Method and Apparatus for Generating Fresh Water, and Method and Apparatus for Desalinating Sea Water

ABSTRACT

An object is to provide a fresh water generating method that is capable of efficiently producing purified water, such as fresh water, from unpurified water, such as sea water. Provided is a fresh water generating method for generating fresh water by way of reverse osmosis membrane filtration, which includes mixing sea water with low salt concentration water having a salt concentration lower than sea water to produce mixed water, and subjecting the mixed water prepared by the mixing to reverse osmosis membrane filtration, thereby generating fresh water.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.12/990,099 entitled “Method and Apparatus for Generating Fresh Water,and Method and Apparatus for Desalinating Sea Water”, filed Nov. 10,2010, which claims priority to International Patent Application No.PCT/JP2009/069932, filed Nov. 26, 2009, Japanese Patent Application No.2008-304623, filed Nov. 28, 2008, Japanese Patent Application No.2009-031819, Feb. 13, 2009, Japanese Patent Application No. 2009-031861,Feb. 13, 2009, Japanese Patent Application No. 2009-032073, Feb. 14,2009, and Japanese Patent Application No. 2009-032075, Feb. 14, 2009,the entire disclosures of each of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for generatingfresh water by way of reverse osmosis membrane filtration, and relatesto a method and apparatus for desalinating sea water by way offiltration using, for example, a reverse osmosis filtration apparatus.

2. Description of Related Art

In recent years, there has been a problem that rain falls locally or ina short period of time due to global warming or the like and hence waterresources are unevenly distributed geographically or temporally, or awater holding capacity of a mountainous area is lowered due to declinein forest industry or deforestation, which leads to difficulty in stablysecuring water resources.

In order to stably secure water resources, there has been a proposeddesalinating sea water by filtration process using a reverse osmosismembrane, for example, in seafront areas (e.g., Japanese PatentApplication Laid-open No. 2008-55317).

SUMMARY OF THE INVENTION

However, a conventional technique for desalinating sea water poses aproblem in that filtration of sea water by a reverse osmosis membranerequires sea water to be pressurized and pressure-fed to a reverseosmosis membrane unit by a pump or the like, and therefore the higherthe salt concentration (salinity) of sea water, the larger the energyrequired.

In consideration of the above problem, it is an object of the presentinvention to provide a method and apparatus for generating fresh water,and a method and apparatus for desalinating sea water that are capableof efficiently producing purified water, such as fresh water, fromunpurified water, such as sea water.

According to one aspect of the present invention, there is provided afresh water generating method for generating fresh water by way ofreverse osmosis membrane filtration, which includes mixing sea waterwith low salt concentration water having a salt concentration lower thansea water to produce mixed water, and subjecting the mixed waterprepared by the mixing to reverse osmosis membrane filtration, therebygenerating fresh water.

According to another aspect of the present invention, there is provideda fresh water generating apparatus for generating fresh water by way ofreverse osmosis membrane filtration, which is configured such that seawater is mixed with low salt concentration water having a saltconcentration lower than sea water to produce mixed water, and the mixedwater produced by the mixing is subjected to reverse osmosis membranefiltration, thereby generating fresh water.

According to another aspect of the present invention, there is provideda sea water desalinating method for desalinating sea water by way offiltration using a reverse osmosis membrane device, which methodcomprises carrying out a mixing step of mixing, as diluent water,biologically treated water produced by biologically treating organicwastewater into sea water to produce mixed water, and a mixed waterprocessing step of feeding the mixed water produced by the mixing stepto the reverse osmosis membrane device, at which the mixed water isfiltered, thereby desalinating the sea water.

According to the sea water desalinating method, the mixed water producedby mixing, as diluent water, biologically treated water having a saltconcentration lower than sea water into sea water is fed to the reverseosmosis membrane device, at which the mixed water is filtered. Whereby,the pressure for pressure-feeding the mixed water to the reverse osmosismembrane device can be lowered than a pressure for pressure-feeding seawater. Thus, an amount of energy required for pressure-feeding per unitquantity of produced fresh water can be saved. Also, permeate flux of amembrane of a reverse osmosis membrane device can be increased, andhence the filtration flow rate can be increased. Furthermore, the loadto the membrane (chemical load due to salt in salt water, and physicalload due to pressure) can be lowered and hence the operation life of themembrane can be extended. Still furthermore, the biologically treatedwater can be effectively utilized.

In the sea water desalinating method using the biologically treatedwater, it is preferable to carry out a wastewater treatment step ofproducing biologically treated water by biologically treating organicwastewater, producing permeate by filtering the biologically treatedwater by using a clarifier that has at least one of a microfiltrationmembrane, a ultrafiltration membrane and a sand filtration means, andproducing permeate that is purified water and concentrated water byfiltering the permeate using a reverse osmosis membrane device, whereinthe biologically treated water used as the diluent water in the mixingstep is the concentrated water.

According to the above sea water desalinating method, purified water canbe recovered in the wastewater treatment step to enable producing anadvantageous effect of more efficiently recovering purified water.

In the sea water desalinating method including the wastewater treatmentstep, it is preferable to carry out filtration with the clarifierinstalled as a submerged membrane below the liquid level of a biologicaltreatment tank for the biological treatment, in the wastewater treatmentstep.

According to the sea water desalinating method, in a case of usingactivated sludge in biological treatment, only the filtrate containinglittle activated sludge can be produced from biologically treated watercontaining activated sludge through the submerged membrane, which isadvantageous in that the concentration of biological species in thebiological treatment tank can be easily increased and hence the volumeof the biological treatment tank can be reduced. Furthermore, incomparison with the arrangement where the clarifier is installed outsideof the biological treatment tank, there are advantageous effects in thatthe system or device used in the sea water desalination method can befurther reduced in size and a passage for returning sludge concentratedat the clarifier to the biological treatment tank can be omitted.

Still furthermore, in the sea water desalinating method using thebiologically treated water, it is preferable to filter the mixed waterusing a clarifier that has at least one of a microfiltration membrane, aultrafiltration membrane and a sand filtration means, prior to thefiltration using the reverse osmosis membrane device, in the mixed watertreatment step.

According to the above sea water desalination method, organic solidmatter can be suppressed from adhering to the membrane surface of thereverse osmosis membrane device, which produces an advantageous effectof producing fresh water with higher efficiency. There is also anadvantageous effect in that fresh water with higher degree of purity canbe produced.

In the sea water desalinating method which includes filtration of mixedwater using the clarifier prior to filtration using the reverse osmosismembrane device in the mixed water treatment step, it is preferable tobiologically treat the mixed water prior to the filtration of the mixedwater using the clarifier in the mixed water treatment step.

According to the above sea water desalinating method, the concentrationof dissoluble organic matter in the mixed water is lowered, which makesit possible to produce advantageous effects of suppressing proliferationof microorganisms generated between the clarifier and the reverseosmosis membrane device and suppressing organic solid matter, such asmicroorganisms, from adhering to the membrane surface of the reverseosmosis membrane device used in the mixed water treatment step, andhence more efficiently producing fresh water. As an additionaladvantageous effect, fresh water with higher degree of purity can beproduced.

In the sea water desalinating method using the biologically treatedwater, it is preferable to have the mixing volume ratio of diluent waterto sea water being 0.1 or more when sea water=1, in the mixing step.

According to the sea water desalinating method, there are advantageouseffects in that the amount of energy required for desalinating sea waterper unit quantity of the produced fresh water can be securely loweredand corrosion of various devices or instruments used in the mixing stepor the mixed water treatment step can be suppressed. Also, as anadditional advantageous effect, biological treatment can be carried outwith good results when the biological treatment is carried out in themixed water treatment step.

Furthermore, in the sea water desalinating method using the biologicallytreated water, it is preferable to filter sea water by using a clarifierand mix the sea water subjected to filtration with diluent water in themixing step.

According to the sea water desalinating method, there is an advantageouseffect in that fresh water with higher degree of purity can be produced.When biologically treated water as diluent water is filtered, theconcentration of solid matter in sea water to be mixed with diluentwater is suppressed, which produces an advantageous effect in that freshwater can be more efficiently produced.

According to still another aspect of the present invention, there isprovided a sea water desalinating apparatus for desalinating sea waterby way of filtration using a reverse osmosis membrane device, whichincludes a mixed water treatment part that mixes, as diluent water,biologically treated water, which is produced by biologically treatingorganic wastewater, into sea water to produce mixed water, and feeds themixed water produced by the mixing to the reverse osmosis membranedevice, at which the mixed water is filtered.

According to yet another aspect of the present invention, there isprovided a sea water desalinating method for desalinating sea water byway of filtration using a reverse osmosis membrane device, which methodcomprises carrying out a mixing step of mixing sedimentation treatedwater that is supernatant water produced by sedimentation and separationof inorganic wastewater into sea water to produce mixed water, and amixed water treatment step of feeding the mixed water produced by themixing step to the reverse osmosis membrane device, at which the mixedwater is filtered, thereby desalinating sea water.

According to the sea water desalinating method, the sedimentationtreated water having a salt concentration lower than sea water is, asdiluent water, mixed into sea water to produce mixed water, which is fedto the reverse osmosis membrane device and filtered. Whereby, thepressure for pressure-feeding the mixed water to the reverse osmosismembrane device can be lowered as compared with the pressure forpressure-feeding sea water, and therefore the amount of energy requiredfor pressure-feeding per unit quantity of the produced fresh water canbe lowered. Since the salt concentration of the fed water that is mixedwater to be fed to the reverse osmosis membrane device is lowered, therecovery rate of the treated water can be increased and the amount ofenergy required for pressure-feeding per unit quantity of the producedfresh water can be lowered. Also, the permeate flux of a membrane of areverse osmosis membrane device can be increased, and hence thefiltration flow rate can be increased. Furthermore, the load to themembrane (chemical load due to salt in salt water, and physical load dueto pressure) can be also lowered and hence the operation life of themembrane can be extended. Still furthermore, the sedimentation treatedwater can be effectively utilized.

In the sea water desalinating method using the sedimentation treatedwater, it is preferable to carry out a wastewater treatment step ofsubjecting inorganic wastewater to precipitation and separation toproduce sedimentation treated water, filtering the sedimentation treatedwater by using a clarifier that has at least one of a sand filtrationmeans, a microfiltration membrane and a ultrafiltration membrane toproduce permeate, and filtering the permeate using a reverse osmosismembrane device to produce permeate that is purified water andconcentrated water, wherein the sedimentation treated water used as thediluent water in the mixing step is the concentrated water.

According to the above sea water desalinating method, purified water canbe recovered in the wastewater treatment step to enable producing anadvantageous effect of more efficiently recovering purified water.

Furthermore, in the sea water desalinating method using thesedimentation treated water, it is preferable to filter the mixed waterby using a clarifier that includes at least one of a sand filtrationmeans, a microfiltration membrane and a ultrafiltration membrane, priorto the filtration using the reverse osmosis membrane device, in themixed water treatment step.

According to the above sea water desalination method, inorganic solidmatter can be suppressed from adhering to the membrane surface of thereverse osmosis membrane device used in the mixed water treatment step,which produces an advantageous effect of more efficiently producingfresh water. As an additional advantageous effect, fresh water withhigher degree of purity can be produced.

In the sea water desalinating method using the sedimentation treatedwater, it is preferable to have the mixing volume ratio of diluent waterto sea water being 0.1 or more when sea water=1.

According to the sea water desalinating method, there are advantageouseffects in that, with respect to the energy required for desalinatingsea water, the amount of energy per unit quantity of the produced freshwater can be securely lowered and corrosion of various devices orinstruments used in the mixing step or the mixed water treatment stepcan be suppressed.

Furthermore, in the sea water desalinating method using thesedimentation treated water, it is preferable to filter sea water usinga clarifier and mix the filtered sea water with diluent water in themixing step.

According to the above sea water desalinating method, there is anadvantageous effect in that fresh water with higher degree of purity canbe produced. When sedimentation treated water as diluent water isfiltered, the concentration of solid matter in the diluent water islowered and the concentration of solid matter in sea water to be mixedwith the diluent water can be suppressed, which produces an advantageouseffect in that fresh water can be more efficiently produced.

According to another aspect of the present invention, there is provideda sea water desalinating apparatus for desalinating sea water by way offiltration using a reverse osmosis membrane device, which includes amixed water treatment part that mixes, as diluent water, sedimentationtreated water that is supernatant water produced by sedimentation andseparation of inorganic wastewater, into sea water to produce mixedwater, and feeds the mixed water produced by the mixing to the reverseosmosis membrane device, at which the mixed water is filtered.

According to still another aspect of the present invention, there isprovided a sea water desalinating method for desalinating sea water byway of filtration using a reverse osmosis membrane device, which methodincludes a mixing step of mixing, as diluent, inorganic wastewater intosea water to produce mixed water, and a mixed water treatment step offeeding the mixed water produced by the mixing step to the reverseosmosis membrane device, at which the mixed water is filtered, therebydesalinating the sea water.

According to the sea water desalinating method, the mixed water producedby mixing, as diluent water, inorganic wastewater having a saltconcentration lower than sea water is fed to the reverse osmosismembrane device, at which the mixed water is filtered. Whereby, apressure for pressure-feeding the mixed water to the reverse osmosismembrane device can be kept lower than the pressure for pressure-feedingsea water. Thus, an amount of energy required for pressure-feeding perunit quantity of produced fresh water can be kept low. Also, the saltconcentration of the fed water that is mixed water to be fed to thereverse osmosis membrane device is lowered such that the recovery rateof treated water can be increased and the amount of energy required forpressure-feeding per unit quantity of the produced fresh water can bekept low. Furthermore, the permeate flux of a membrane of a reverseosmosis membrane device can be increased, and hence the filtration flowrate can be increased. Furthermore, the load to the membrane (chemicalload due to salt in salt water, and physical load due to pressure) canbe lowered and hence the operation life of the membrane can be extended.

According to yet another aspect of the present invention, there isprovided a sea water desalinating apparatus for desalinating sea waterby way of filtration using a reverse osmosis membrane device, whichincludes a mixed water treatment part that mixes, as diluent water,inorganic wastewater into sea water to produce mixed water, and feedsthe mixed water produced by the mixing to a reverse osmosis membranedevice, at which the mixed water is filtered.

According to another aspect of the present invention, there is provideda fresh water generating apparatus that includes a first treatment partthat separates low salt concentration wastewater having a saltconcentration lower than sea water into permeate and concentrated waterby way of reverse osmosis membrane filtration, and a second treatmentpart that mixes the concentrated water produced at the first treatmentpart to produce mixed water and separates the mixed water into permeateand concentrated water by way of reverse osmosis membrane filtration,thereby producing fresh water separated as the permeate respectively atthe first and second treatment parts, wherein the first treatment partincludes a first salt concentration measurement means for measuring thesalt concentration of the low salt concentration wastewater, such thatthe amount of permeate produced at the first treatment part and theamount of permeate produced at the second treatment part are controlledbased on the measured value by the first salt concentration measurementmeans.

In the above fresh water generating apparatus, low salt concentrationwastewater is utilized as fresh water resource at the first treatmentpart, and therefore the energy for generating fresh water can be savedby an amount corresponding to this utilization as compared with the casein which only see water is utilized as a resource of fresh water.

Since sea water can be diluted at the second treatment part, the saltconcentration can be lowered. Also, in this point of view, fresh watercan be generated with small energy consumption.

Since even sea water is utilized as a resource of fresh water, resourcesof fresh water can be stably secured, and in a case in which the saltconcentration of the low salt concentration wastewater has been changed,the generating amounts at the first treatment part and the secondtreatment part are controlled such that the total generating amount canbe stabilized.

In the fresh water generating apparatus including the salt concentrationmeasurement means, it is preferable that control is made such that thegenerating amount at the first treatment part is increased while thegenerating amount at the second treatment part is decreased, when themeasured value is not more than, or less than a predetermined referencevalue.

With the above arrangement, when the measured value of the saltconcentration is not more than, or less than a predetermined referencevalue, a greater amount of fresh water can be produced with the sameenergy by increasing the recovery rate, as compared with a case in whichthe measured value is within the reference range.

Thus, the generating amount (amount of fresh water) at the secondtreatment part, which requires high energy, can be decreased by anamount corresponding to increase in generating amount at the firsttreatment part. Thus, fresh water can be efficiently generated with thesame energy.

According to still another aspect of the present invention, there isprovided a fresh water generating apparatus that includes a firsttreatment part that separates low salt concentration wastewater having asalt concentration lower than sea water into permeate and concentratedwater by way of reverse osmosis membrane filtration, and a secondtreatment part that mixes, as diluent water, the concentrated waterproduced at the first treatment part into sea water to produce mixedwater and separates the mixed water into permeate and concentrated waterby way of reverse osmosis membrane filtration, thereby producingpermeate as fresh water separated respectively at the first and secondtreatment parts, wherein the first treatment part includes a flow ratemeasurement means for measuring the inflow rate of the low saltconcentration wastewater flown into the first treatment part, such thatthe filtration rate at the first treatment part and the filtration rateat the second treatment part are controlled based on the measured valueby the flow rate measurement means.

In the fresh water generating apparatus, low salt concentrationwastewater is utilized as fresh water resource at the first treatmentpart, and therefore the energy for generating fresh water can be savedby an amount corresponding to this utilization as compared with the casein which only see water is utilized as a resource of fresh water.

Since sea water can be diluted at the second treatment part, the saltconcentration can be lowered. Also, in this point of view, fresh watercan be generated with small energy consumption.

Furthermore, even when the amount of intake of the low saltconcentration wastewater is decreased, the control can be made such thatthe filtration rate at the first treatment part is decreased while thefiltration rate at the second treatment part at which sea water isutilized as a resource of fresh water. Contrarily, even when the amountof intake of the low salt concentration wastewater is increased, thecontrol can be made such that the filtration rate at the first treatmentpart is increased while the filtration rate at the second treatment partis decreased. Thus, the amount of fresh water generated can bestabilized without the necessity to provide a huge space forinstallation of an excessively large storage tank.

Also, it is possible to prevent, for example, the case in which low saltconcentration wastewater must be disposed of. Thus, low saltconcentration wastewater, from which fresh water can be generated at lowcost, can be satisfactorily utilized, and fresh water can be efficientlyproduced.

In the fresh water generating apparatus including the flow ratemeasurement means, it is preferable that the first treatment part andthe second treatment part each include plural reverse osmosis membraneunits for carrying out reverse osmosis membrane filtration, and thenumber of the reverse osmosis membrane units for carrying out reverseosmosis membrane filtration at the first treatment part and the secondtreatment part can be controlled based on the measured value by the flowrate measurement means.

In the fresh water generating apparatus, in which the number of thereverse osmosis membrane units to carry out reverse osmosis membranefiltration at each of the treatment parts is controlled, the filtrationrate of each of the treatment parts can be easily controlled.

Furthermore, in the above structure, the control is preferably made suchthat, when the measured value is increased, the number of the reverseosmosis membrane units to carry out reverse osmosis membrane filtrationat the first treatment part is increase while the number of the reverseosmosis membrane units to carry out reverse osmosis membrane filtrationat the second treatment part is decreased.

With the thus structured fresh water generating apparatus, even when thelow salt concentration wastewater flown into the apparatus is increased,the increased low salt concentration wastewater can be satisfactorilyutilized as a resource of fresh water by increasing the number of thereverse osmosis membrane units of the first treatment part, while theamount of sea water to be treated, which treatment is costly, can bedecreased by decreasing the number of the reverse osmosis membrane unitsof the second treatment part. Thus, a predetermined amount of freshwater can be efficiently produced.

According to another aspect of the present invention, there is provideda fresh water generating apparatus that includes a first treatment partthat separates low salt concentration wastewater having a saltconcentration lower than sea water into permeate and concentrated waterby way of reverse osmosis membrane filtration, and a second treatmentpart that mixes, as diluent water, the concentrated water produced atthe first treatment part into sea water to produce mixed water andseparate the mixed water into permeate and concentrated water by way ofreverse osmosis membrane filtration, wherein a part of low saltconcentration wastewater of the first treatment part is bypassed to befed as diluent water into sea water at the second treatment part,thereby producing permeate as fresh water separated respectively at thefirst and second treatment parts, wherein the first treatment partincludes a flow rate measurement means for measuring the flow rate oflow salt concentration wastewater flown into the first treatment part,such that the amount of the low salt concentration wastewater to bebypassed can be controlled based on the measured value by the flow ratemeasurement means.

In the fresh water generating apparatus, when the amount of lowconcentration wastewater flown is large, a part thereof is bypassed soas to be able to be utilized to dilute sea water, and thereby the saltconcentration of sea water at the second treatment part can be lowered.As a result, the power cost required for reverse osmosis membranefiltration at the second treatment part can be reduced.

Furthermore, according to still another aspect of the present invention,there is provided a fresh water generating method that includes carryingout a first treatment step of separating low salt concentrationwastewater having a salt concentration lower than sea water intopermeate and concentrated water by way of reverse osmosis membranefiltration, and a second treatment step of mixing, as diluent water, theconcentrated water produced in the first treatment step into sea waterto produce mixed water and separate the mixed water into permeate andconcentrated water by way of reverse osmosis membrane filtration,thereby producing permeate as fresh water separated respectively in thefirst and second treatment steps,

wherein the amount of low salt concentration wastewater to be treated ismeasured, such that the filtration rate of the first treatment step andthe filtration rate of the second treatment step are controlled based onthe measured value.

Alternatively, there is provided a fresh water generating method thatincludes carrying out a first treatment step of separating low saltconcentration wastewater having a salt concentration lower than seawater into permeate and concentrated water by way of reverse osmosismembrane filtration, and a second treatment step of mixing, as diluentwater, the concentrated water produced at the first treatment part intosea water to produce mixed water and separate the mixed water intopermeate and concentrated water by way of reverse osmosis membranefiltration, thereby producing permeate as fresh water separatedrespectively in the first and second treatment steps,

wherein the amount of low salt concentration wastewater to be treated ismeasured, and control is made such that a part of the low saltconcentration wastewater is mixed into sea water to dilute the sea waterin the second treatment step.

According to the present invention, purified water, such as fresh watercan be efficiently produced from non-purified water, such as sea water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a sea water desalinatingapparatus according to one embodiment of the present invention;

FIG. 2 is a schematic block diagram of a sea water desalinatingapparatus according to another embodiment of the present invention;

FIG. 3 is a schematic block diagram of a sea water desalinatingapparatus according to still another embodiment of the presentinvention;

FIG. 4 is a schematic block diagram of a sea water desalinatingapparatus according to yet another embodiment of the present invention;

FIG. 5 is a schematic block diagram of a sea water desalinatingapparatus according to another embodiment of the present invention;

FIG. 6 is a schematic block diagram of a sea water desalinatingapparatus according to still another embodiment of the presentinvention;

FIG. 7 is a schematic diagram of a second biological treatment tank andthe inside of the tank;

FIG. 8 is a schematic block diagram of a sea water desalinatingapparatus according to another embodiment of the present invention;

FIG. 9 is a schematic block diagram of a sea water desalinatingapparatus according to another embodiment of the present invention;

FIG. 10 is a schematic block diagram of a sea water desalinatingapparatus according to still another embodiment of the presentinvention;

FIG. 11 is a schematic block diagram of a sea water desalinatingapparatus according to yet another embodiment of the present invention;

FIG. 12 is a schematic block diagram of a fresh water generatingapparatus according to one embodiment of the present invention;

FIG. 13 is a schematic block diagram of a fresh water generatingapparatus according to another embodiment of the present invention;

FIG. 14 is a schematic block diagram of a fresh water generatingapparatus according to still another embodiment of the presentinvention;

FIG. 15 is a schematic block diagram of a fresh water generatingapparatus according to yet another embodiment of the present invention;

FIG. 16 is a schematic block diagram of a sea water desalinatingapparatus of Test Example 1;

FIG. 17 shows the result of Test Example 1;

FIG. 18 is a schematic block diagram of a sea water desalinatingapparatus of Example 1;

FIG. 19 is a schematic block diagram of a sea water desalinatingapparatus of Comparative Example 1;

FIG. 20 is a schematic block diagram of a sea water desalinatingapparatus of Test Example 3;

FIG. 21 shows the result of Test Example 3;

FIG. 22 is a schematic block diagram of a sea water desalinatingapparatus of Example 2; and

FIG. 23 is a schematic block diagram of a sea water desalinatingapparatus of Comparative Example 2.

BRIEF DESCRIPTION OF THE INVENTION

Now, the description will be made for an embodiment of the presentinvention with reference to the drawings attached hereto.

First Embodiment

Now, the description will be made for a sea water desalinating apparatusas a fresh water generating apparatus of a first embodiment, and a seawater desalinating method as a fresh water generating method.

Meanwhile, in a conventional sea water desalinating method, sea watermust be pressurized and pressure-fed to a reverse osmosis membranedevice by a pump or the like in order to subject the sea water tofiltration by the reverse osmosis membrane device, which presents aproblem in that the higher the salt concentration of sea water, thelarger the energy required.

In addition to the above issue regarding sea water, wastewatercontaining organic matter represented by, for example, sewage water(hereinafter referred also to as “organic wastewater”) is generallysubjected to biological treatment. Biologically treated water producedby biologically treating this organic wastewater is released to sea orriver in the current circumstances, and hence little organic wastewateris efficiently utilized.

In consideration of the above problem, an object of the first embodimentis to provide a sea water desalinating method and a sea waterdesalinating apparatus that are capable of efficiently producingpurified water, such as fresh water, while utilizing biologicallytreated water produced by biologically treating organic wastewater.

First, the description will be made for a sea water desalinatingapparatus of the first embodiment.

FIG. 1 is a schematic block diagram of the sea water desalinatingapparatus of the first embodiment.

A sea water desalinating apparatus 1 of the first embodiment, as shownin FIG. 1, includes a biological treatment part 3 for biologicallytreating organic wastewater B by biological species, a mixed watertreatment part 2 for mixing, as diluent water, the biologically treatedwater produced at the biological treatment part 3 into sea water A,feeding the mixed water produced by the mixing to a first osmosismembrane device 23, at which the mixed water is filtered, therebyproducing fresh water C that is permeate and concentrated water D, and amethane fermentation part 4 for producing methane by fermenting thebiological species proliferated by biological treatment at thebiological treatment part 3.

The sea water desalinating apparatus 1 of the first embodiment isconfigured so as to transfer the sea water A to the mixed watertreatment part 2, the organic wastewater B to the biological treatmentpart 3, and the biologically treated water to the mixed water treatmentpart 2, the proliferated biological species to the methane fermentationpart 4, and the concentrated water D to a concentrated water storagetank (not shown), respectively.

The sea water desalinating apparatus 1 of the first embodiment isconfigured to recover the fresh water C that is permeate.

The biological treatment is a treatment to decompose organic mattercontained in water by biological species, such as bacteria, protozoa andmetazoan. A specific example of the biological treatment includesaeration using activated sludge.

The sea water A is water containing salt, and for example, water havinga salt concentration of 1.0 to 8.0% by mass, and more specifically,water having a salt concentration of 2.5 to 6.0% by mass.

The sea water A is not herein necessarily limited to water in the sea,and is intended to include water in land area, such as water of lake(salt lake, brackish lake), water of swamps, and water of pond, as longas they are water having a salt concentration of 1.0% by mass or more.

The organic wastewater B is wastewater containing organic matter, andfor example, wastewater having a BOD (Biochemical Oxygen Demand), as anindex of organic matter concentration, of 2000 mg/L or lower, and morespecifically wastewater having a BOD of about 200 mg/L. The organicwastewater B is water having a salt concentration lower than the seawater A. The organic wastewater B is, for example, wastewater having asalt concentration ratio relative to the sea water A of 1:not more than0.1, and more specifically 1:not more than 0.01.

Examples of the organic wastewater B include sewage water (e.g.,domestic wastewater or rainwater flowing into sewage pipes), andindustrial wastewater (wastewater discharged from, such as a foodfactory, a chemical factory, a factory in electronics industry, and apulp plant).

The mixed water treatment part 2 is configured to mix, as diluent water,biologically treated water produced at the biological treatment part 3into the sea water A.

The mixed water treatment part 2 includes a first biological treatmenttank 21 for biologically treating the mixed water produced by themixing, a first clarifier 22 that has at least one of a microfiltrationmembrane (MF membrane) and an ultrafiltration membrane (UF membrane) andclarifies the mixed water, which has been subjected to the biologicaltreatment at the biological treatment tank 21, by way of filtration tothereby produce first permeate and first concentrated water, and a firstreverse osmosis membrane device 23 that filters the mixed water that isthe first permeate to thereby produce the fresh water C that is secondpermeate and second concentrated water.

The mixed water treatment part 2 is configured such that biologicallytreated water produced at the biological treatment part 3 is mixed, asdiluent water, into the sea water A to produce mixed water; the mixedwater produced by the mixing is transferred to the first biologicaltreatment tank 21 to be biologically treated by the first biologicaltreatment tank 21; the mixed water subjected to the biological treatmentis transferred to the first clarifier 22 to be filtered by the firstclarifier 22, thereby producing first permeate and first concentratedwater; and the first concentrated water is transferred to a concentratedwater storage tank (not shown) while the mixed water that is the firstpermeate is transferred to the first reverse osmosis membrane device 23to be filtered by the first reverse osmosis membrane device 23, therebyproducing the fresh water C that is second permeate and secondconcentrated water.

By the clarifying is herein meant rougher filtration than reverseosmosis membrane filtration, that is, a treatment carried out prior tothe filtration by a reverse osmosis membrane device and made to removeimpurities (e.g., solid matter or the like) coarser than those filteredby a reverse osmosis membrane.

The sea water desalinating apparatus 1 of the first embodiment isconfigured to recover the fresh water C that is the second permeate.

The first reverse osmosis membrane device 23 is of the type that areverse osmosis membrane (RO membrane) is contained in a pressurevessel.

The mixed water treatment part 2 includes a first pump 24 forpressurizing the first permeate and pressure-feeding the same to thefirst reverse osmosis membrane device 23, such that the secondconcentration water is pressure-fed from the first reverse osmosismembrane device 23 by pressure-feeding the first permeate to the firstreverse osmosis membrane device 23 via the first pump 24.

The mixed water treatment part 2 includes a firstscale-prevention-solution feeding means (not shown) for feeding a scaleprevention solution, which contains a scale prevention agent (an agentcapable of suppressing the formation of scale on the RO membrane), tothe RO membrane of the first reverse osmosis membrane device 23.

Examples of the scale prevention agent include a carboxylic acidpolymer, a carboxylic acid polymer blended product and a phosphonate.

The mixed water treatment part 2 further includes a firstmembrane-cleaning-solution feeding means (not shown) for feeding amembrane cleaning solution, which contains a membrane solution agent (anagent capable of dissolving original substances of crud capable ofadhering to a membrane), to the RO membrane of the first reverse osmosismembrane device 23.

No limitation is intended to the material of the membrane cleaningagent, and examples of the membrane cleaning agent include variouschemicals, such as an acid, an alkali, an oxidizing agent, a chelatingagent and a surface active agent. Examples of the acid include anorganic acid (e.g., citric acid, oxalic acid, etc.), an inorganic acid(e.g., hydrochloric acid, sulphuric acid, nitric acid, etc.). An exampleof the alkali includes sodium hydroxide. Examples of the oxidizing agentinclude hydrogen peroxide and sodium hypochlorite.

As the membrane cleaning solution, a mixed liquid with two or more kindsof membrane cleaning agents mixed together (e.g., mixture of sodiumhydroxide and a surface active agent) may be used.

The mixed water treatment part 2 includes a water turbine 25 that ispowered by pressure of the second concentrated water which has beenpressure-fed from the first reverse osmosis membrane device 23, and themixed water treatment part 2 is configured to be capable of beingpowered upon driving of the water turbine 25 by the pressure of thesecond concentrated water, which is effected by transferring the secondconcentrated water pressure-fed from the first reverse osmosis membranedevice 23 to the water turbine 25.

The sea water desalinating apparatus 1 of the first embodiment isconfigured to transfer the second concentrated water, which has beenused for driving the water turbine 25, to a concentrated water storagetank (not shown).

The first clarifier 22 is of the type to be installed outside of thefirst biological treatment tank 21.

The mixed water treatment part 2 includes a secondmembrane-cleaning-solution feeding means (not shown) for feeding theaforesaid membrane cleaning solution to a membrane of the firstclarifier 22.

The biological treatment part 3 includes a second biological treatmenttank 31 for biologically treating organic wastewater to producebiologically treated water, a second clarifier 32 that has at least oneof a microfiltration membrane (MF membrane) and an ultrafiltrationmembrane (UF membrane) and is configured to filter the biologicallytreated water produced at the second biological treatment tank 31 toproduce third permeate and third concentrated water, and a secondreverse osmosis membrane device 33 for filtering biologically treatedwater that is third permeate to produce purified water E that is fourthpermeate, and biologically treated water that is fourth concentratedwater.

The second clarifier 32 is installed as a submerged membrane below theliquid level of the second biological treatment tank 31.

The biological treatment part 3 includes a fourthmembrane-cleaning-solution feeding means (not shown) for feeding theaforesaid membrane cleaning solution to a membrane of the secondbiological treatment tank 31.

The sea water desalinating apparatus 1 of the first embodiment isconfigured to transfer the organic wastewater B to the second biologicaltreatment tank 31.

The biological treatment part 3 is configured to biologically treat thetransferred organic wastewater B at the second biological treatment tank31 to produce biologically treated water, filter the biologicallytreated water by using the second clarifier 32 to produce the thirdpermeate and the third concentrated water, transfer the third permeateto the second reverse osmosis membrane device 33, and filter the thirdpermeate by using the second reverse osmosis membrane device 33 toproduce the purified water E that is the fourth permeate, andbiologically treated water that is the fourth concentrated water.

The sea water desalinating apparatus 1 of the first embodiment isconfigured to transfer the third concentrated water to the methanefermentation part 4, and transfer, as diluent water, the biologicallytreated water that is the fourth concentrated water to the mixed watertreatment part 2, and recover the fourth permeate as the purified waterE.

The second reverse osmosis membrane unit 33 is of the type that areverse osmosis membrane is contained in a pressure vessel.

By the RO membrane of the second reverse osmosis membrane unit 33 of thefirst embodiment is intended to include a nano-filtration membrane (NFmembrane).

The biological treatment part 3 is configured to feed the third permeateto the second reverse osmosis membrane device 33 after pressurizing thesame via a second pump 34.

The biological treatment part 3 includes a secondscale-prevention-solution means (not shown) for feeding the aforesaidscale prevention solution to the RO membrane of the second reverseosmosis membrane device 33.

The biological treatment part 3 includes a thirdmembrane-cleaning-solution feeding means (not shown) for feeding themembrane cleaning solution to the RO membrane of the second reverseosmosis membrane device 33.

The sea water desalinating apparatus 1 of the first embodiment isconfigured to transfer a membrane cleaning solution (referred also to as“used membrane cleaning solution”) used for cleaning the membrane to atleast one of the first biological treatment tank 21 and the secondbiological treatment tank 31, when the membrane cleaning agent is anacid, an alkali, a chelating agent, a surface active agent or the like.According to the needs and circumstances, the sea water desalinatingapparatus 1 of the first embodiment includes a membrane-cleaning-agentneutralization means (not shown) for neutralizing the used membranecleaning solution before transferring the used membrane cleaningsolution to the biological treatment tank(s). Themembrane-cleaning-agent neutralization means is configured to neutralizethe used membrane cleaning solution by adding and mixing an acid or analkali to the used membrane cleaning solution. Themembrane-cleaning-agent neutralization means is configured to allow thepH of the neutralized membrane cleaning solution to be preferably in arange of 5 to 9 and more preferably in a range of 6 to 8.

Furthermore, when the membrane-cleaning-agent is an oxidizing agent, thesea water desalinating apparatus 1 of the first embodiment is configuredto mix together and dehydrate the used membrane cleaning solution andthe third concentrated water, transfer solid matter generated by thedehydration, as third concentrated water, to the methane fermentationpart 4, and transfer, as biologically treated water, aqueous solution(supernatant water) generated by the dehydration to the secondbiological treatment tank 31, according to the needs and circumstances.

The methane fermentation part 4 is configured to produce methane byfermenting biological species contained in the third concentrated water,which is water with biological species concentrated therein, whichbiological species having been proliferated by the biological treatmentat the biological treatment part 3, by anaerobic microorganisms, such asacid generating bacteria and methane generating bacteria.

The sea water desalinating apparatus 1 of the first embodiment includesa steam power production part (not shown) that performs steam powerproduction by combustion of methane produced at the methane fermentationpart 4.

The sea water desalinating apparatus 1 of the first embodiment isconfigured to increase the temperature of biologically treated waterwithin the biological treatment tank by waste heat, such as steamgenerated at the steam power production part. Furthermore, the sea waterdesalinating apparatus 1 of the first embodiment is configured toincrease the temperature of objective water to be transferred to themembrane system for membrane treatment by the aforesaid waste heat.

The sea water desalinating apparatus 1 of the first embodiment includesa concentration difference power production part 5 that generates powerby utilizing the difference between the salt concentration of the secondconcentrated water and the salt concentration of the third permeate.

The concentration difference power production part 5 includes a tank 51,a semi-permeable membrane 54 for dividing the inside of the tank 51 intotwo sections.

The concentration difference power production part 5 further includes athird permeate accommodation part 52 for accommodation of the thirdpermeate and a second concentrated water accommodation part 53 foraccommodation of the second concentrated water.

The third permeate accommodation part 52 and the second concentratedwater accommodation part 53 are formed by dividing the inside of thetank 51 into the two sections by the semi-permeable membrane 54.

The sea water desalinating apparatus 1 of the first embodiment isconfigured to transfer a part of the third permeate to the thirdpermeate accommodation part 52 and transfer the second concentratedwater to the second concentrated water accommodation part 53 beforetransferring to a concentrated water storage tank (not shown).

The concentration difference power production part 5 is configured togenerate power by utilizing the height difference in liquid level causedby the increase in the liquid level of the third permeate accommodationpart 52, which height difference is in turn caused by the transfer ofonly the water content of the second concentrated water via thesemi-permeable membrane 54 to the third permeate accommodation part 52,which transfer is in turn caused by the difference in salt concentrationbetween the second concentrated water and the third permeate.

The sea water desalinating apparatus 1 of the first embodiment isconfigured to transfer, as the concentrated water D, the secondconcentrated water used at the concentration difference power productionpart 5 and the water content of the third permeate to a concentratedwater storage tank (not shown), and recover, as industrial water F, thethird permeate used at the concentration difference power productionpart 5 and remained in the third permeate accommodation part 52.

The concentration difference power production part 5 may be configuredto generate power by using the purified water E or the fresh water C inplace of the third permeate. That is, the concentration difference powerproduction part 5 may include a purified water accommodation part foraccommodation of the purified water E or a fresh water accommodationpart for accommodation of the fresh water C. In this case, the sea waterdesalinating apparatus 1 of the first embodiment is configured totransfer the purified water E or the fresh water C to the concentrationdifference power production part 5.

Now, the description will be made for a sea water desalinating method ofthe first embodiment.

The sea water desalinating method of the first embodiment includescarrying out a mixing step of mixing, as diluent water, biologicallytreated water produced by biologically treating organic wastewater intosea water, and a mixed water treatment step of feeding the mixed waterproduced by the mixing step to a reverse osmosis membrane device, atwhich the mixed water is filtered.

Specifically, the sea water desalinating method of the first embodimentis a method of desalinating sea water A by carrying out a wastewatertreatment step of biologically treating organic wastewater B within theinside of the second biological treatment tank 31 to producebiologically treated water, filtering the biologically treated water byusing the second clarifier 32 to produce third permeate and thirdconcentrated water, and filtering biologically treated water that is thethird permeate by using the second reverse osmosis membrane device 33 toproduce fourth permeate and biologically treated water that is fourthconcentrated water, a mixing step of mixing, as the aforesaid diluentwater, biologically treated water that is the fourth concentrated waterinto the sea water A to produce mixed water, and a mixed water treatmentstep of biologically treating the mixed water produced by the mixingstep within the first biological treatment tank 21 to producebiologically treated water, then filtering the biologically treatedwater by using the first clarifier 22 to provide first permeate andfirst concentrated water, and filtering the mixed water that is thefirst permeate by using the first reverse osmosis membrane device 23 toprovide second permeate and second concentrated water.

In the mixing step, the mixing volume ratio of the sea water A to thediluent water is preferably 1 to 0.1 or more, and more preferably 1 to 1or more, in order to make the dilution effect significant.

The sea water desalinating method of the first embodiment isadvantageous in the fact that, by having the mixing volume ratio of thesea water A to the diluent water being 1 to 0.1 or more, the saltconcentration can be lowered and the amount of energy required fordesalinating the sea water A per unit quantity of the produced freshwater can be securely saved, and corrosion of various devices orinstruments used in the mixing step or the mixed water treatment step.Furthermore, there is another advantageous effect in that biologicaltreatment in the mixed water treatment step can be carried out with goodresults.

In the sea water desalinating method of the first embodiment, the saltconcentration of the mixed water is preferably 3.0% by mass or lower,and more preferably 1.8% by mass or lower. Furthermore, in the sea waterdesalinating method of the first embodiment, the salt concentration ofthe diluent water is preferably one third or less of the saltconcentration of the sea water A to be diluted with diluent water, andmore preferably one tenth or less of the salt concentration of the seawater A to be diluted with diluent water. The sea water desalinatingmethod of the first embodiment is also advantageous in that, by havingthe salt concentration of diluent water being one third or less of thesalt concentration of the sea water A to be diluted with diluent water,the fresh water C with higher degree of purity can be produced.

The sea water desalinating apparatus of the first embodiment and the seawater desalinating method of the first embodiment configured asmentioned above produce the following advantageous effects.

According to the sea water desalinating method of the first embodiment,which includes carrying out the mixing step of mixing, as diluent water,biologically treated water having a salt concentration lower than thesea water A into the sea water A, and the mixed water treatment step offeeding the mixed water produced by carrying out the mixing step to thefirst reverse osmosis membrane device 23, at which the mixed water isfiltered, thereby desalinating the sea water A, the pressure forpressure-feeding the mixed water to the first reverse osmosis membraneunit 23 can be kept lower than the pressure for pressure-feeding the seawater A. Whereby, the amount of energy required for pressure-feeding perunit quantity of produced fresh water C can be saved. Also, the permeateflux of a membrane of the first reverse osmosis membrane device 23 canbe increased, and hence the filtration flow rate can be increased.Furthermore, the load to the membrane of the first reverse osmosismembrane device 23 (chemical load due to salt in the sea water A, andphysical load due to pressure) can be lowered and hence the operationlife of the membrane can be extended. Still furthermore, thebiologically treated water can be effectively utilized.

According to the sea water desalinating method of the first embodiment,the filtration of the mixed water is made using the first clarifier 22prior to the filtration using the first reverse osmosis membrane device23 in the mixed water treatment step. Whereby, it is possible tosuppress organic solid substance or salt from adhering onto the membranesurface of the first reverse osmosis membrane device 23, and henceproduce an advantageous effect in that fresh water C can be moreefficiently produced. There is also an advantageous effect in that thefresh water C with higher degree of purity can be produced.

According to the sea water desalinating method of the first embodiment,the biological treatment of the mixed water is made prior to thefiltration of the mixed water using the first clarifier 22 in the mixedwater treatment step. Whereby, the concentration of dissoluble organicsubstance in the mixed water is reduced, which makes it possible toproduce advantageous effects of suppressing proliferation ofmicroorganisms generated between the first clarifier 22 and the firstreverse osmosis membrane device 23 and suppressing organic solid matter,such as microorganisms, from adhering to the membrane surface of thefirst reverse osmosis membrane device 23, and hence more efficientlyproducing the fresh water C. As an additional advantageous effect, thefresh water C with higher degree of purity can be produced.

According to the sea water desalinating method of the first embodiment,purified water E can be recovered in a wastewater treatment step bycarrying out the wastewater treatment step of biologically treatingorganic wastewater within the second biological treatment tank 31 toproduce biologically treated water, then filtering the biologicallytreated water using the second clarifier 32 to produce the thirdpermeate and the third concentrated water, and then filtering the thirdpermeate using the second reverse osmosis membrane device 33 to producethe fourth permeate and the fourth concentrated water. Thus, there is anadvantageous effect in that purified water can be more efficientlyrecovered.

According to the sea water desalinating apparatus 1 of the firstembodiment, the second clarifier 32 is installed as a submerged membranebelow the liquid level of the second biological treatment tank 31.Whereby, when activated sludge is used for biological treatment, it ispossible to produce, through the submerged membrane, only the filtratecontaining little activated sludge from biologically treated watercontaining activated sludge. Therefore, there are advantageous effectsin that the concentration of biological species in the second biologicaltreatment tank 31 can be increased and the volume of the secondbiological treatment tank 31 can be reduced. As additional advantageouseffects, the sea water desalinating apparatus 1 can be further reducedin size as compared with the arrangement in which the second clarifier32 is installed outside of the biological treatment tank, andfurthermore it is possible to omit a passage for returning sludgeconcentrated in the second clarifier 32 to the second biologicaltreatment tank 31.

By having the sea water desalinating apparatus 1 of the first embodimentconfigured to feed the first permeate to the first reverse osmosismembrane device 23 after pressurizing the first permeate via the firstpump 24 to produce the second concentrated water, and to be powered bydriving the water turbine 25 by the pressure of the second concentratedwater, there is an advantageous effect in that an energy can beproduced. Furthermore, when the produced energy is utilized in a step ofproducing purified water from sea water or sewage water, there is anadvantageous effect in that purified water can be more efficientlyrecovered.

By having the sea water desalinating apparatus 1 of the first embodimentconfigured to include the methane fermentation part 4 capable ofproducing methane by fermenting biological species proliferated bybiological treatment at the biological treatment part 3, there is anadvantageous effect in that an energy can be produced. Furthermore, whenthe produced energy is utilized in a step of producing purified waterfrom sea water or sewage water, there is an advantageous effect in thatpurified water can be more efficiently recovered. As an additionaladvantageous effect, excessive biological species can be disposed whilebeing effectively utilized.

By having the sea water desalinating apparatus 1 of the first embodimentconfigured to include the steam power production part such that thetemperature of the biologically treated water is increased in thebiological treatment tank by waste heat, such as steam generated at thesteam power production part, it is possible to increase the temperatureof biologically treated water within the biological treatment tank to ahigh temperature at which biological species are high in activity, whenthe temperature is low and biological species are low in activity withinactivated sludge, especially during the winter season. Thus, there areadvantageous effects in that purified water can be more efficientlyrecovered while at the same time effectively utilizing the producedenergy.

By having the sea water desalinating apparatus 1 of the first embodimentconfigured to include the steam power production part such that thetemperature of to-be-treated water transferred to the membrane systemfor membrane treatment is increased by waste heat, such as steamgenerated at the steam power production part, the viscosity of theto-be-treated water is lowered and hence the permeate flux of theto-be-treated water is easily increased. Thus, there is an advantageouseffect in that purified water can be more efficiently recovered.

By having the sea water desalinating apparatus 1 of the first embodimentconfigured to include the concentration difference power production part5 that produces power by utilizing the difference between the saltconcentration of the second concentrated water, which is higher in saltconcentration than the mixed water, and the salt concentration of thethird permeate, there is an advantageous effect in that energy can beproduced. When this produced energy is utilized in a step of producingpurified water from sea water or sewage water, there is an advantageouseffect in that purified water can be more efficiently recovered.

Furthermore, by having the sea water desalinating apparatus 1 of thefirst embodiment configured to include the firstscale-prevention-solution feeding means and the secondscale-prevention-solution feeding means, there is an advantageous effectin that scale, which may be able to be generated on the reverse osmosismembrane of the first reverse osmosis membrane device 23 and the reverseosmosis membrane of the second reverse osmosis membrane device 33, canbe suppressed. Thus, there is an advantageous effect in that purifiedwater can be more efficiently recovered.

By having the sea water desalinating apparatus 1 of the first embodimentconfigured to transfer the used membrane cleaning solution to thebiological treatment tank when the membrane cleaning solution is anacid, an alkali, a chelating agent or a surface active agent, there isan advantageous effect in that organic matter contained in the usedmembrane cleaning solution can be decomposed within the biologicaltreatment tank, and therefore the organic matter of the used membranecleaning solution is not needed to be separately decomposed.

By having the sea water desalinating apparatus 1 of the first embodimentconfigured, when the membrane-cleaning-agent is an oxidizing agent, tomix together and dehydrate the used membrane cleaning solution and thethird concentrated water, transfer, as the third concentrated water,solid matter generated by the dehydration to the methane fermentationpart 4, and transfer, as the biologically treated water, aqueoussolution (supernatant water) generated by the dehydration to the secondbiological treatment tank 31, there is an advantageous effect in that itis possible to decompose organic matter contained in the used membranecleaning solution in the biological treatment tank while inhibitingdeath of biological species by an oxidizing agent, and hence omit thenecessity to separately decompose organic matter of the used membranecleaning solution.

While the sea water desalinating apparatus of the first embodiment andthe sea water desalinating method of the first embodiment present theabove advantageous effects, the sea water desalinating apparatus of thepresent invention and the sea water desalinating method of the presentinvention are not necessarily limited to the above arrangements, and maybe modified according to the needs and circumstances.

For example, in the sea water desalinating apparatus 1 of the firstembodiment, the second clarifier 32 is installed as a submerged membranebelow the liquid level of the second biological treatment tank 31.However, as shown in FIG. 2, the second clarifier 32 may be of the typethat is installed outside of the second biological treatment tank 31. Inthis case, the sea water desalinating apparatus 1 of the presentinvention is configured to transfer biologically treated water that hasbeen biologically treated at the second biological treatment tank 31 tothe second clarifier 32.

In the sea water desalinating apparatus 1 of the first embodiment, thefirst clarifier 22 is of the type that is installed outside of the firstbiological treatment tank 21. However, the first clarifier 22 may be ofthe type that is installed as a submerged membrane below the liquidlevel of the first biological treatment tank 21.

The sea water desalinating apparatus 1 of the first embodiment includesthe first scale-prevention-solution feeding means and the secondscale-prevention-solution feeding means. However, the sea waterdesalinating apparatus 1 may include only the secondscale-prevention-solution feeding means while not including the firstscale-prevention-solution feeding means, in which a scale preventionsolution fed to the second reverse osmosis membrane device 33 by thesecond scale-prevention-solution feeding means is discharged, as thefourth concentrated water, from the second reverse osmosis membranedevice 33, and the scale prevention solution is fed to the first reverseosmosis membrane device 23.

According to the thus configured sea water desalinating apparatus 1 ofthe first embodiment, the scale prevention solution is difficult topermeate through a reverse osmosis membrane, and thus a scale preventionsolution used at the second reverse osmosis membrane device 33 can beutilized at the first reverse osmosis membrane device 23, as well, andpower required for feeding the scale prevention solution can be saved,which produces an advantageous effect in that purified water can be moreefficiently recovered.

In this case, the sea water desalinating apparatus 1 of the presentinvention may be configured such that a scale prevention solutiondischarged, as the fourth concentrated water, from the second reverseosmosis membrane device 33 is fed to the first reverse osmosis membranedevice 23 via the first biological treatment tank 21, the firstclarifier 22 and the like, or may be configured such that the scaleprevention solution is fed directly to the first reverse osmosismembrane device 23 without passing the first biological treatment tank21, the first clarifier 22 and the like. Especially, according to thesea water desalinating apparatus 1 of the present invention, in whichthe scale prevention solution is fed directly to the first reverseosmosis membrane device 23 without passing the first biologicaltreatment tank 21, the first clarifier 22 and the like, there is anadvantageous effect in that the scale prevention solution is suppressedfrom being diluted at the first biological treatment tank 21, the firstclarifier 22 and the like, thereby the scale prevention solution isefficiently fed to the first reverse osmosis membrane device 23, andthus purified water can be more efficiently recovered.

According to the sea water desalinating method of the first embodiment,in the mixed water treatment step, the biological treatment of the mixedwater using the first biological treatment tank 21 is carried out andthe filtration of the mixed water subjected to the biological treatmentusing the first clarifier 22 is carried out, prior to the filtrationusing the first reverse osmosis membrane device 23. However, the seawater desalinating method of the present invention may be configuredsuch that the biological treatment of the mixed water by the firstbiological treatment tank 21 and the filtration of the mixed water bythe first clarifier 22 are not carried out.

In the above form of the sea water desalinating method of the presentinvention, as shown in FIGS. 3 and 4, it is preferable to employ anarrangement, in which, prior to the mixing of biologically treated waterthat is the fourth concentrated water, as diluent water, into the seawater A, the sea water A is filtered using a third clarifier 10 that hasat least one of a microfiltration membrane (MF membrane) and anultrafiltration membrane (UF membrane) to produce fifth permeate andfifth concentrated water, and the sea water A that is the fifth permeateis mixed with the diluent water to produce mixed water.

According to the sea water desalinating method, there is an advantageouseffect in that fresh water C with higher degree of purity can beproduced. There is another advantageous effect in that, whenbiologically treated water as diluent water has been filtered, theconcentration of solid matter in the diluent water is lowered and theconcentration of solid matter in the sea water A to be mixed with thediluent water is kept low, and thereby the fresh water C can be moreefficiently produced.

In the sea water desalinating method of the present invention, the fifthconcentrated water can be treated as concentrated water similar to thefirst concentrated water.

In the sea water desalinating method of the first embodiment, the thirdpermeate produced at the second clarifier 32 is filtered using thesecond reverse osmosis membrane device 33 in the wastewater treatmentstep. However, it is possible to employ an arrangement in whichfiltration of the third permeate by the second reverse osmosis membranedevice 33 is not carried out.

In the above form of the sea water desalinating method of the presentinvention, as shown in FIGS. 5 and 6, it is preferable to employ anarrangement in which, prior to the mixing of biologically treated waterthat is the third permeate, as diluent water, into the sea water A, thesea water A is filtered using the third clarifier 10 that has at leastone of a microfiltration membrane (MF membrane) and an ultrafiltrationmembrane (UF membrane), and the sea water A filtered using the thirdclarifier 10 is mixed with, as diluent water, biologically treated waterthat is the third permeate to produce mixed water.

In the sea water desalinating method of the first embodiment, biologicalspecies proliferated at the biological treatment part 3 are fermented bythe methane fermentation part 4 to produce methane, while it is possibleto subject the biological species to another treatment, such asdehydration, in the sea water desalinating method of the presentinvention.

In the first embodiment, the first clarifier 22 is configured to filtermixed water transferred to the first clarifier 22 by at least one of amicrofiltration membrane (MF membrane) and an ultrafiltration membrane(UF membrane), while it may be configured to filter the mixed water bysand filtration means having a sand filter. The first embodiment isconfigured in this matter to produce an advantageous effect in thatimpurities of a large amount of water can be removed with low power.

In the arrangement in which sand filtration is carried out, the firstclarifier 22 may be configured to carry out sand filtration by onestage, or by two or more stages.

By the stage of sand filtration is meant the number of sand filtersconnected in tandem.

In the arrangement in which sand filtration is carried out, the firstclarifier 22 may be configured to further filter mixed water, which hasbeen subjected to sand filtration, by at least one of a microfiltrationmembrane (MF membrane) and an ultrafiltration membrane (UF membrane).

In the arrangement in which the first clarifier 22 is to carry out sandfiltration, the first clarifier 22 is provided with a cleaning means(not shown) for cleaning a sand filter layer.

In the first embodiment, the second clarifier 32 is configured to filterbiologically treated water, which has been transferred to the secondclarifier 32, by at least one of a microfiltration membrane (MFmembrane) and an ultrafiltration membrane (UF membrane), while thesecond clarifier 32 may be configured such that biologically treatedwater is subjected to solid-liquid separation in a sedimentation tankand the biologically treated water subjected to solid-liquid separationis filtered by a sand filtration means.

In the arrangement in which sand filtration is carried out, the secondclarifier 32 may be configured to carry out sand filtration by onestage, or by two or more stages.

In the arrangement in which sand filtration is carried out, the secondclarifier 32 may be configured such that the biologically treated watersubjected to sand filtration is further filtered by at least one of amicrofiltration membrane (MF membrane) and an ultrafiltration membrane(UF membrane).

In the first embodiment, the second clarifier 32 may be configured suchthat biologically treated water is subjected to solid-liquid separationin a sedimentation tank, and the biologically treated water subjected tosolid-liquid separation is filtered by at least one of a microfiltrationmembrane (MF membrane) and an ultrafiltration membrane (UF membrane).

In the arrangement in which the second clarifier 32 is to carry out sandfiltration, the second clarifier 32 is provided with a cleaning means(not shown) for cleaning a sand filter layer.

When the first embodiment includes the third clarifier 10, in the firstembodiment, the third clarifier 10 is configured to have the sea waterA, which is transferred to the third clarifier 10, filtered by at leastone of a microfiltration membrane (MF membrane) and an ultrafiltrationmembrane (UF membrane), while the third clarifier 10 may be configuredto have the sea water A filtered by a sand filtration means.

In the arrangement in which sand filtration is carried out, the thirdclarifier 10 may be configured to carry out the sand filtration by onestage, or by two or more stages.

In the arrangement in which sand filtration is carried out, the thirdclarifier 10 may be configured such that sea water, which has beenfiltered by the sand filtration means, is further filtered by least oneof a microfiltration membrane (MF membrane) and an ultrafiltrationmembrane (UF membrane).

In the arrangement in which the third clarifier 10 is to carry out sandfiltration, it is provided with a cleaning means (not shown) forcleaning a sand filter layer.

In the first embodiment, it is possible to employ an arrangement inwhich power generation is made by utilizing natural energies (e.g., wavepower, tidal power, wind power, solar power and geothermal sources), andthe power thus produced from natural energies is utilized as a drivingpower for a pump or the like of the sea water desalinating apparatus 1of the first embodiment. The first embodiment is advantageous in that itis possible to suppress generation of gasses, such as carbon dioxide,which may be able to cause influences on the environment, hold inrunning out of fossil fuels, or prevent risks, such as nuclearaccidents, by utilizing power produced from natural energies.

The sea water desalinating apparatus 1 of the first embodiment isprovided with the water turbine 25 in the mixed water treatment part 2,while a pressure converter (a pressure recovery device) for convertingthe pressure of the second concentrated water, which has beenpressure-fed from the first reverse osmosis membrane device 23, to thepressure for transferring mixed water directly (without intervention ofelectricity) to the first reverse osmosis membrane device 23, may beprovided in place of the water turbine 25.

When the pressure converter is provided, the sea water desalinatingapparatus 1 of the first embodiment is configured such that the secondconcentrated water, which has been pressure-fed from the first reverseosmosis membrane device 23, is transferred to the pressure converter,and the second concentrated water used at the pressure converter istransferred to a concentrated water storage tank. The sea waterdesalinating apparatus 1 of the first embodiment is configured such thatmixed water is transferred to the pressure converter before it passesthrough the first pump 24, and mixed water pressurized at the pressureconverter is transferred to the first reverse osmosis membrane device 23via the first pump 24.

The sea water desalinating apparatus 1 of the first embodiment thusconfigured is advantageous in that the power of the first pump 24 can besaved.

The sea water desalinating apparatus 1 of the first embodiment isconfigured such that the third concentrated water is transferred to themethane fermentation part 4, while the sea water desalinating apparatus1 may be provided with a solubilization means that decomposes, dissolvesand solubilizes biological species (when biological species arecontained in activated sludge, activated sludge is also meant asbiological species) contained in the third concentrated water by, forexample, chemicals (an alkali, an acid, an oxidizing agent or the like),ultrasonic waves, heat, and microorganisms having a capability ofsolubilizing activated sludge.

In the case of providing the solubilization means, the sea waterdesalinating apparatus 1 of the first embodiment is configured such thatthe third concentrated water is transferred to the solubilization means,and the third concentrated water that is a solubilized liquid istransferred to the methane fermentation part 4. In the case in whichsolubilization is made by chemicals, the sea water desalinatingapparatus 1 of the first embodiment is configured such that asolubilized liquid is adjusted in pH to close to neutral (e.g., pH 6-8)according to the needs and circumstances, and the third concentratedwater that is the solubilized liquid with its pH adjusted is transferredto the methane fermentation part 4.

The first embodiment thus configured is advantageous in that biologicalspecies are decomposed by the solubilization means and therefore thebiological species can be easily decomposed by anaerobic microorganisms(methane generating bacteria, etc.).

As chemicals used by the solubilization means, chemicals (an alkali, anacid, an oxidizing agent) used for cleaning a membrane of a reverseosmosis membrane or the like are preferable. In the first embodiment,when the chemicals used as the solubilization means are chemicals usedfor the cleaning, there is an advantageous effect in that the necessityto separately subject the used chemicals to a treatment making themnon-hazardous can be reduced.

The sea water desalinating apparatus 1 of the first embodiment mayinclude a hydro extractor that separates methane fermentation digestionliquid, which has been produced by the methane fermentation ofbiological species of the third concentrated water at the methanefermentation part 4, into dehydration cake and supernatant water, and anincineration equipment for incineration of the dehydration cake.

In the case of providing the hydro extractor and the incinerationequipment, the sea water desalinating apparatus 1 of the firstembodiment is configured such that the methane fermentation digestionliquid is transferred to the hydro extractor, the dehydration cake istransferred to the incineration equipment, and the supernatant water istransferred, as biologically treated water, to the second biologicaltreatment tank 31. The sea water desalinating apparatus 1 of the firstembodiment is preferably configured to include the solubilization meansso as to enable the third concentrated water that is the solubilizedliquid to be transferred to the methane fermentation part 4. The seawater desalinating apparatus 1 of the first embodiment thus configuredenables biological species to be decomposed by the solubilization meansto be thereby easily decomposable by anaerobic microorganisms (methanegenerating bacteria, etc.), and thereby improves the decompositionefficiency of biological species by anaerobic microorganisms. Therefore,in the sea water desalinating apparatus 1 of the first embodiment, theamount of solid content in the methane fermentation digestion liquid iskept low, and hence the amount of dehydration cake to be incinerated atthe incineration equipment is kept low, which produces an advantageouseffect in that incineration costs at the incineration equipment can bekept low.

In the case of providing the solubilization means, the sea waterdesalinating apparatus 1 of the first embodiment may be configured suchthat solubilized liquid is transferred, as biologically treated water,to the second biological treatment tank 31.

In the case in which the sea water desalinating apparatus 1 of the firstembodiment is configured to carry out biological treatment usingactivated sludge within the second biological treatment tank 31, asshown in FIG. 7, a carrier 35 that coagulates activated sludge may bedisposed within the second biological treatment tank 31.

In the case in which the carrier 35 is disposed within the secondbiological treatment tank 31, the sea water desalinating apparatus 1 ofthe first embodiment is configured such that coagulated activated sludgethat is activated sludge coagulated by the carrier 35 and separated fromthe carrier 35 is formed, and furthermore the coagulated activatedsludge and organic wastewater are mixed together to produce biologicallytreated water. The sea water desalinating apparatus 1 of the firstembodiment includes an aeration means 36 that aerates the water insideof the second biological treatment tank 31.

By providing the sea water desalinating apparatus 1 of the firstembodiment with the carrier 35, activated sludge is coagulated and itssedimentation rate increases. Accordingly, the sedimentation separationcharacteristics of activated sludge are enhanced so that there is anadvantageous effect in that the membrane separation characteristics ofbiologically treated water can be improved.

The carrier 35 includes trapping members 35 a to which the activatedsludge adheres, and a supporting member 35 b for supporting the trappingmembers 35 a. The carrier 35 is configured such that the trappingmembers 35 a sway with flows generated by aeration of the aeration means36.

The supporting member 35 b has a filament shape. The supporting member35 b is disposed so as to have the axis of the filament orientedsubstantially perpendicular to the surface of the water within thesecond biological treatment tank 31. The supporting member 35 b issecured within the second biological treatment tank 31.

No limitation is intended to the material of the supporting member 35 b,as long as it supports the trapping members 35 a, but examples of thematerial of the supporting member 35 b include polyester, acrylic resin,polyethylene, carbon fiber and the like.

The trapping members 35 a each have a filament shape.

No limitation is intended to the material of the trapping members 35 a,as long as the activated sludge can easily adhere thereto, but examplesof the material of the trapping members 35 a include acrylic resin,polyester, polyethylene, carbon fiber, and the like.

The sea water desalinating apparatus 1 of the first embodiment may beconfigured to include an ozone-adding device (not shown) for addingozone to biologically treated water that is fourth concentrated waterproduced by the filtration using the second reverse osmosis membranedevice 33, such that the biologically treated water, as diluent water,to which ozonation has been carried out, is mixed into the sea water A.

The sea water desalinating apparatus 1 of the first embodiment thusconfigured enables the concentration of organic matter contained in thediluent water to be lowered, and hence the concentration of organicmatter contained in mixed water produced by mixing the diluent waterinto the sea water A to be lowered. Thus, the sea water desalinatingapparatus 1 of the first embodiment can suppress organic solid matterfrom adhering onto the membrane surface of the first reverse osmosismembrane device 23, and thereby can efficiently increase the permeateflux of a membrane of the first reverse osmosis membrane device 23,which results in producing an advantageous effect in that the freshwater C can be more efficiently produced.

The sea water desalinating apparatus 1 of the first embodiment thusconfigured enables reduction of odor components contained in diluentwater by ozone, and sterilization of microorganisms contained in diluentwater, which produces an advantageous effect in that fresh water C withhigher degree of purity can be produced.

According to the first embodiment, it is possible to efficiently producepurified water, such as fresh water, while utilizing biologicallytreated water produced by biologically treating organic wastewater.

Second Embodiment

Now, the description will be made for a sea water desalinating apparatusas a fresh water generating apparatus, and a sea water desalinatingmethod as a fresh water generating method, of a second embodiment.

Meanwhile, in a conventional sea water desalinating method, sea watermust be pressurized by a pump or the like and pressure fed to a reverseosmosis membrane device in order to carry out filtration of sea water bythe reverse osmosis membrane device, which poses a problem in that thehigher the salt concentration of sea water, the larger the energyrequired.

On the other hand, in addition to the above issue regarding sea water,wastewater containing inorganic matter, such as metal, represented by,for example, wastewater from a factory for manufacturing metal, such assteel (hereinafter referred also to as “inorganic wastewater”) isgenerally subjected to pretreatment, such as pH adjustment to besolidified, and then subjected to sedimentation separation.Sedimentation treated water that is supernatant water produced bysedimentation and separation of this inorganic wastewater is currentlyreleased to the oceans or rivers, which poses a problem in that a largeamount of water not efficiently utilized exists.

In consideration of the above problem, an object of the secondembodiment is to provide a sea water desalinating method and a sea waterdesalinating apparatus that are capable of efficiently producingpurified water, such as fresh water, while utilizing inorganicwastewater.

First, the description will be made for a sea water desalinatingapparatus of the second embodiment.

FIG. 8 is a schematic block diagram of the sea water desalinatingapparatus of the second embodiment. As shown in FIG. 8, a sea waterdesalinating apparatus 201 of the second embodiment includes asedimentation treatment part 203 that subjects inorganic wastewater 200Bto sedimentation and separation (hereinafter referred also to as“sedimentation treatment”) to produce sedimentation treated water thatis supernatant water and concentrated water 200D containing a largeamount of solid matter, and a mixed water treatment part 202 that mixes,as diluent water, sedimentation treated water that is supernatant waterproduced at the sedimentation treatment part 203 into sea water 200A andfeeds the mixed water produced by the mixing to a reverse osmosismembrane device 223 to filter the same, thereby producing fresh water200C that is permeate and concentrated water 200D.

The sea water desalinating apparatus 201 of the second embodiment isconfigured such that the sea water 200A is transferred to the mixedwater treatment part 202, the inorganic wastewater 200B to thesedimentation treatment part 203, the sedimentation treated water to themixed water treatment part 202, and the concentrated water 200D to aconcentrated water storage tank (not shown).

Furthermore, the sea water desalinating apparatus 201 of the secondembodiment is configured to recover fresh water 200C that is theaforesaid permeate.

The sea water 200A is water containing salt, and for example, has a saltconcentration of about 1.0 to 8.0% by mass, and more specifically has asalt concentration of 2.5 to 6.0% by mass.

The sea water 200A is not herein necessarily limited to water in thesea, and is intended to include water in land area, such as water oflake (salt lake, brackish lake), water of swamps, and water of pond, aslong as they are water having a salt concentration of 1.0% by mass ormore.

The inorganic wastewater 200B is wastewater containing inorganic matterand having a low concentration of organic matter, and, for example,wastewater having a BOD (Biochemical Oxygen Demand) of 50 mg/L or lower,and preferably wastewater having a BOD of 10 mg/L or lower.

The inorganic wastewater 200B is water having a salt concentration lowerthan the sea water 200A. The inorganic wastewater 200B is, for example,wastewater having a salt concentration relative to the sea water 200A of1:not more than 0.1, and more specifically 1:not more than 0.01.

Examples of the inorganic wastewater 200B include industrial wastewater(e.g., wastewater discharged from various factories, such as a steelfactory, a chemical factory and a factory in electronics).

The mixed water treatment part 202 is configured such that sedimentationtreated water produced by the sedimentation treatment part 203 is mixed,as diluent water, into the sea water 200A to produce mixed water.

The mixed water treatment part 202 includes a first clarifier 222 thathas at least one of a microfiltration membrane (MF membrane) and aultrafiltration membrane (UF membrane) and clarifies the mixed water byway of filtration to produce first permeate and first concentratedwater, and a first reverse osmosis membrane device 223 that filtersmixed water that is the first permeate to produce fresh water 200C thatis second permeate and second concentrated water.

The mixed water treatment part 202 is configured such that sedimentationtreated water produced at the sedimentation treatment part 203 is mixed,as diluent water, into the sea water 200A to produce mixed water, themixed water produced by the mixing is transferred to the first clarifier222 to be filtered by the first clarifier 222 to produce first permeateand first concentrated water, the first concentrated water istransferred to a concentrated water storage tank (not shown), mixedwater that is the first permeate is transferred to the first reverseosmosis membrane device 223 to be treated by the first reverse osmosismembrane device 223 to produce the fresh water 200C that is secondpermeate and the second concentrated water.

By the clarifying is herein meant rougher filtration than reverseosmosis membrane filtration, that is, a treatment carried out prior tothe filtration by the reverse osmosis membrane device and made to removeimpurities (e.g., solid matter or the like) coarser than those filteredby a reverse osmosis membrane.

The sea water desalinating apparatus 201 of the second embodiment isconfigured to recover the fresh water 200C that is the second permeate.

The first reverse osmosis membrane device 223 is of the type that areverse osmosis membrane (RO membrane) is contained in a pressurevessel.

The mixed water treatment part 202 includes a first pump 224 forpressurizing the first permeate and pressure-feeding the same to thefirst reverse osmosis membrane device 223, such that the secondconcentrated water is pressure-fed to the first reverse osmosis membranedevice 223 by pressure-feeding the first permeate to the first reverseosmosis membrane device 223 via the first pump 224.

The mixed water treatment part 202 includes a firstscale-prevention-solution feeding means (not shown) for feeding a scaleprevention solution, which contains a scale prevention agent (agentcapable of suppressing the formation of scale on the RO membrane), tothe RO membrane of the first reverse osmosis membrane device 223.

Examples of the scale prevention agent include a carboxylic acidpolymer, a carboxylic acid polymer blended product and a phosphonate.

The mixed water treatment part 202 also includes a firstmembrane-cleaning-solution feeding means for feeding a membrane cleaningsolution, which contains a membrane solution agent (agent capable ofdissolving original substances of crud capable of adhering to amembrane), to the RO membrane of the first reverse osmosis membranedevice 223.

No limitation is intended to the material of the membrane cleaningagent, and examples of the membrane cleaning agent include an acid, analkali, an oxidizing agent, a chelating agent and a surface activeagent. Examples of the acid include an organic acid (e.g., citric acid,oxalic acid, etc.), an inorganic acid (e.g., hydrochloric acid,sulphuric acid, nitric acid, etc.). An example of the alkali includessodium hydroxide. Examples of the oxidizing agent include hydrogenperoxide and sodium hypochlorite.

As the membrane cleaning solution, a mixed liquid with two or more kindsof membrane cleaning agents mixed together may be used (e.g., mixture ofsodium hydroxide and a surface active agent).

The mixed water treatment part 202 includes a water turbine 225 that ispowered by pressure of the second concentrated water that has beenpressure-fed from the first reverse osmosis membrane device 223, and isconfigured to be capable of being powered upon the driving of the waterturbine 225 by the pressure of the second concentrated water, which iseffected by transferring the second concentrated water pressure-fed fromthe first reverse osmosis membrane device 223 to the water turbine 225.

The sea water desalinating apparatus 201 of the second embodiment isconfigured to transfer the second concentrated water, which has beenused for driving the water turbine 225, to a concentrated water storagetank (not shown).

The first clarifier 222 is of the type to be installed outside of thetank.

The mixed water treatment part 202 includes a secondmembrane-cleaning-solution feeding means (not shown) for feeding theaforesaid membrane cleaning solution to the membrane of the firstclarifier 222.

The sedimentation treatment part 203 includes a sedimentation separationtank 231 for subjecting the inorganic wastewater 200B to sedimentationand separation to produce sedimentation treated water that issupernatant water and concentrated water 200D, a second clarifier 232that has at least one of a microfiltration membrane (MF membrane) and anultrafiltration membrane (UF membrane) and is configured to filter thesedimentation treated water produced at the sedimentation separationtank 231 to produce third permeate and third concentrated water, and asecond reverse osmosis membrane device 233 that filters sedimentationtreated water that is the third permeate to produce purified water 200Ethat is fourth permeate and sedimentation treated water that is fourthconcentrated water.

The sea water desalinating apparatus 201 of the second embodiment isconfigured to include an aggregation-agent adding means for adding anaggregation agent to the sedimentation separation tank 231 to subjectthe inorganic wastewater 200B to aggregation, sedimentation andseparation by the aggregation agent, according to the needs andcircumstances.

The second clarifier 232 is of the type to be installed outside of thesedimentation separation tank 231.

The sedimentation treatment part 203 includes a fourthmembrane-cleaning-solution feeding means (not shown) for feeding theaforesaid membrane cleaning solution to the membrane of the secondclarifier 232.

The sea water desalinating apparatus 201 of the second embodiment isconfigured to transfer the inorganic wastewater 200B to thesedimentation separation tank 231.

The sedimentation treatment part 203 is configured to subject thetransferred inorganic wastewater 200B to sedimentation and separation bythe sedimentation separation tank 231 to produce sedimentation treatedwater that is supernatant water and concentrated water 200D, transferthe sedimentation treated water to the second clarifier 232, transferthe concentrated water 200D to a concentrated water storage tank (notshown), filter the sedimentation treated water using the secondclarifier 232 to produce third permeate and third concentrated water,transfer the third permeate to the second reverse osmosis membranedevice 233, and filter the third permeate using the second reverseosmosis membrane device 233 to produce purified water 200E that isfourth permeate and sedimentation treated water that is fourthconcentrated water.

The sea water desalinating apparatus 201 of the second embodiment isconfigured to transfer, as diluent water, sedimentation treated waterthat is the fourth concentrated water to the mixed water treatment part202, and recover the fourth permeate as the purified water 200E.

The second reverse osmosis membrane device 233 is of the type that areverse osmosis membrane is contained in a pressure vessel.

An RO membrane of the second reverse osmosis membrane device 233 of thesecond embodiment is intended to include a nano-filtration membrane (NFmembrane), as well.

The sedimentation treatment part 203 is configured to feed the thirdpermeate to the second reverse osmosis membrane device 233 afterpressurizing the third permeate via a second pump 234.

The sedimentation treatment part 203 includes a secondscale-prevention-solution means (not shown) for feeding the aforesaidscale prevention solution to the RO membrane of the second reverseosmosis membrane device 233.

The sedimentation treatment part 203 includes a thirdmembrane-cleaning-solution feeding means (not shown) for feeding theaforesaid membrane cleaning solution to the RO membrane of the secondreverse osmosis membrane device 233.

The sea water desalinating apparatus 201 of the second embodimentincludes a concentration difference power production part 205 thatgenerates power by utilizing the difference between the saltconcentration of the second concentrated water and the saltconcentration of the third permeate.

The concentration difference power production part 205 includes a tank251, and a semi-permeable membrane 254 for dividing the inside of thetank 251 into two sections.

The concentration difference power production part 205 further includesa third permeate accommodation part 252 for accommodation of the thirdpermeate and a second concentrated water accommodation part 253 foraccommodation of the second concentrated water.

The third permeate accommodation part 252 and the second concentratedwater accommodation part 253 are formed by dividing the inside of thetank 251 into the two sections with the semi-permeable membrane 254.

The sea water desalinating apparatus 201 of the second embodiment isconfigured to transfer a part of the third permeate to the thirdpermeate accommodation part 252, and transfer the second concentratedwater to the second concentrated water accommodation part 253 beforetransferring the second concentrated water to a concentrated waterstorage tank (not shown).

The concentration difference power production part 205 is configured togenerate power by utilizing the height difference in liquid level causedby the increase in the liquid level of the third permeate accommodationpart 252, which height difference is in turn caused by the transfer ofonly the water content of the second concentrated water via thesemi-permeable membrane 254, which transfer is in turn caused by thedifference in salt concentration between the second concentrated waterand the third permeate.

The sea water desalinating apparatus 201 of the second embodiment isconfigured to transfer, as the concentrated water 200D, the secondconcentrated water used at the concentration difference power productionpart 205 and the water content of the third permeate transferred via thesemi-permeable membrane 254, to a concentrated water storage tank (notshown), and recover, as industrial water 200F, the third permeate usedat the concentration difference power production part 205 and remainedin the third permeate accommodation part 252.

The concentration difference power production part 205 may be configuredto generate power by using the purified water 200E or the fresh water200C in place of the third permeate. That is, the concentrationdifference power production part 205 may include a fresh wateraccommodation part for accommodation of the purified water 200E or afresh water accommodation part for accommodation of the fresh water200C, in place of the third permeate accommodation part 252. In thiscase, the sea water desalinating apparatus 201 of the second embodimentis configured to transfer the purified water 200E or the fresh water200C to the concentration difference power production part 205.

Now, the description will be made for a sea water desalinating method ofa second embodiment.

The sea water desalinating method of the second embodiment includescarrying out a mixing step of mixing, as diluent water, sedimentationtreated water that is supernatant water produced by subjecting inorganicwastewater to sedimentation and separation into sea water, and a mixedwater treatment step of feeding the mixed water produced by the mixingstep to a reverse osmosis membrane apparatus, at which the mixed wateris filtered.

Specifically, the sea water desalinating method of the second embodimentis a method of desalinating sea water by carrying out a wastewatertreatment step of subjecting the inorganic wastewater 200B tosedimentation and separation within the sedimentation separation tank231 to produce sedimentation treated water that is supernatant water,further filtering the sedimentation treated water using the secondclarifier 232 to produce third permeate and third concentrated water,and then filter sedimentation treated water that is the third permeateusing the second reverse osmosis membrane device 233 to produce fourthpermeate and sedimentation treated water that is fourth concentratedwater, a mixing step of mixing, as the diluent water, sedimentationtreated water that is the fourth concentrated water, into the sea water200A to produce mixed water, and a mixed water treatment step offiltering the mixed water produced by the mixing step using the firstclarifier 222 to produce first permeate and first concentrated water,and then filtering mixed water that is the first permeate using thefirst reverse osmosis membrane device 223 to produce second permeate andsecond concentrated water.

In the mixing step, the mixing volume ratio of the sea water 200A to thediluent water is preferably 1 to 0.1 or more, and more preferably 1 to 1or more, in order to make the dilution effect significant.

The sea water desalinating method of the second embodiment isadvantageous in the fact that by having the mixing volume ratio of thesea water 200A to the diluent water being 1 to 0.1 or more, the saltconcentration can be lowered and the amount of energy required fordesalinating the sea water 200A per unit quantity of the produced freshwater 200C can be securely saved, and corrosion of various devices orinstruments used in the mixing step or the mixed water treatment stepcan be suppressed.

In the sea water desalinating method of the second embodiment, the saltconcentration of the mixed water is preferably 3.0% by mass or lower,and more preferably 1.8% by mass or lower. Furthermore, in the sea waterdesalinating method of the second embodiment, the salt concentration ofthe diluent water is preferably one third or less of the saltconcentration of the sea water 200A to be diluted with diluent water,and more preferably one tenth of the salt concentration of the sea water200A to be diluted with diluent water. The sea water desalinating methodof the second embodiment is also advantageous in that, by setting thesalt concentration of diluent water to be one third or less of the saltconcentration of the sea water 200A to be diluted with diluent water,the fresh water 200C with higher degree of purity can be produced.

The sea water desalinating apparatus of the second embodiment and thesea water desalinating method of the second embodiment configured asmentioned above produce the following advantageous effects.

According to the sea water desalinating method of the second embodiment,which carries out the mixing step of mixing, as diluent water,sedimentation treated water having a salt concentration lower than thesea water 200A into the sea water 200A, and the mixed water treatmentstep of feeding the mixed water produced by the mixing step to the firstreverse osmosis membrane device 223, at which the mixed water isfiltered, thereby desalinating the sea water 200A, it is possible tosave an amount of energy required for pressure-feeding per unit quantityof produced fresh water 200C, since a pressure for pressure-feeding themixed water to the first reverse osmosis membrane device 223 can be keptlower than the pressure for pressure-feeding sea water. Also, thepermeate flux of a membrane of the first reverse osmosis membrane device223 can be increased, and hence the filtration flow rate can beincreased. Furthermore, the load to the membrane of the first reverseosmosis membrane device 223 (chemical load due to salt in sea water, andphysical load due to pressure) can be lowered and hence the operationlife of the membrane can be extended. Still furthermore, thesedimentation treated water can be effectively utilized.

According to the sea water desalinating method of the second embodiment,the filtration of the mixed water is made using the first clarifier 222prior to the filtration using the first reverse osmosis membrane device223 in the mixed water treatment step. Whereby, it is possible tosuppress inorganic solid substance or salt from adhering onto themembrane surface of the first reverse osmosis membrane device 223, andhence produce an advantageous effect in that the fresh water 200C can bemore efficiently produced. There is also an advantageous effect in thatthe fresh water 200C with higher degree of purity can be produced.

According to the sea water desalinating method of the second embodiment,purified water 200E can be recovered in a wastewater treatment step bycarrying out the wastewater treatment step of subjecting inorganicwastewater 200B to sedimentation and separation within the sedimentationseparation tank 231 to produce sedimentation treated water that issupernatant water, then filtering the sedimentation treated water usingthe second clarifier 232 to produce the third permeate and the thirdconcentrated water, then filtering the third permeate using the secondreverse osmosis membrane device 233 to produce the fourth permeate andthe fourth concentrated water. Thus, there is an advantageous effect inthat purified water can be more efficiently recovered.

By having the sea water desalinating apparatus 201 of the secondembodiment configured to feed the first permeate to the first reverseosmosis membrane device 223 after pressurizing the first permeate viathe first pump 224 to produce the second concentrated water, and to bepowered by driving the water turbine 225 by the pressure of the secondconcentrated water, there is an advantageous effect in that an energycan be produced. Furthermore, when the produced energy is utilized in astep of producing purified water from the sea water 200A or theinorganic wastewater 200B, there is an advantageous effect in thatpurified water can be more efficiently recovered.

By having the sea water desalinating apparatus 201 of the secondembodiment configured to include the concentration difference powerproduction part 205 that produces power by utilizing the differencebetween the salt concentration of the second concentrated water, whichhas a salt concentration higher than mixed water, and the saltconcentration of the third permeate, there is an advantageous effect inthat an energy can be produced. When this produced energy is utilized ina step of producing purified water from the sea water 200A or theinorganic wastewater 200B, there is an advantage effect in that purifiedwater can be more efficiently recovered.

Furthermore, by having the sea water desalinating apparatus 201 of thesecond embodiment configured to include the firstscale-prevention-solution feeding means and the secondscale-prevention-solution feeding means, there is an advantageous effectin that scale, which may be able to be generated on the reverse osmosismembrane of the first reverse osmosis membrane device 223 and thereverse osmosis membrane of the second reverse osmosis membrane device233, can be suppressed, and therefore purified water can be moreefficiently recovered.

While the sea water desalinating apparatus of the second embodiment andthe sea water desalinating method of the second embodiment produce theabove advantageous effects, the sea water desalinating method of thepresent invention and the sea water desalinating method of the presentinvention are not necessarily limited to the above embodiments, and maybe modified according to the needs and circumstances.

For example, the sea water desalinating apparatus 201 of the secondembodiment includes the first scale-prevention-solution feeding meansand the second scale-prevention-solution feeding means. However, the seawater desalinating apparatus 201 may include only the secondscale-prevention-solution feeding means while not including the firstscale-prevention-solution feeding means, in which a scale preventionsolution fed to the second reverse osmosis membrane device 233 by thesecond scale-prevention-solution feeding means is discharged, as thefourth concentrated water, from the second reverse osmosis membranedevice 233, and the scale prevention solution is fed to the firstreverse osmosis membrane device 223.

According to the thus configured sea water desalinating apparatus 201 ofthe second embodiment, the scale prevention solution is difficult topermeate through a reverse osmosis membrane, and thus a scale preventionsolution used at the second reverse osmosis membrane device 233 can beutilized at the first reverse osmosis membrane device 223, as well, andpower required for feeding the scale prevention solution can be saved,which produces an advantageous effect in that purified water can be moreefficiently recovered.

In this case, the sea water desalinating apparatus 201 of the presentinvention may be configured such that a scale prevention solutiondischarged, as the fourth concentrated water, from the second reverseosmosis membrane device 233 is fed to the first reverse osmosis membranedevice 223 via the first clarifier 222, or may be configured such thatthe scale prevention solution is fed directly to the first reverseosmosis membrane device 223 without intervention of the first clarifier222. Especially, according to the sea water desalinating apparatus 201of the present invention, in which the scale prevention solution is feddirectly to the first reverse osmosis membrane device 223 withoutintervention of the first clarifier 222, there is an advantageous effectin that the scale prevention solution is suppressed from being dilutedat the first clarifier 222, thereby the scale prevention solution isefficiently fed to the first reverse osmosis membrane device 223, andthus purified water can be more efficiently recovered.

In the sea water desalinating method of the second embodiment, in themixed water treatment step, the filtration of the mixed water using thefirst clarifier 222 is carried out, prior to the filtration using thefirst reverse osmosis membrane device 223. However, the sea waterdesalinating method of the present invention may be configured such thatthe filtration of the mixed water by the first clarifier 222 is notcarried out.

In the above form of the sea water desalinating method of the presentinvention, as shown in FIG. 9, it is preferable to employ anarrangement, in which, prior to the mixing of sedimentation treatedwater that is the fourth concentrated water, as diluent water, into thesea water 200A, the sea water 200A is filtered using a third clarifier210 that has at least one of a microfiltration membrane (MF membrane)and an ultrafiltration membrane (UF membrane) to produce fifth permeateand fifth concentrated water, and the sea water 200A that is the fifthpermeate is mixed with the diluent water to produce mixed water, and thefifth permeate in the form of the sea water 200A is mixed with thediluent water to produce mixed water.

In the sea water desalinating method of the present invention, the fifthconcentrated water can be treated as concentrated water similar to thefirst concentrated water.

In the sea water desalinating method of the second embodiment, the thirdpermeate produced at the second clarifier 232 is filtered using thesecond reverse osmosis membrane device 233 in the wastewater treatmentstep. However, it is possible to employ an arrangement in whichfiltration of the third permeate by the second reverse osmosis membranedevice 233 is not carried out.

In the above form of the sea water desalinating method of the presentinvention, as shown in FIG. 10, it is preferable to employ anarrangement in which, prior to the mixing of the sea water 200A with, asdiluent water, sedimentation treated water that is the third permeate,the sea water 200A is filtered using the third clarifier 210 that has atleast one of a microfiltration membrane (MF membrane) and anultrafiltration membrane (UF membrane), and the sea water 200A filteredusing the third clarifier 210 is mixed with, as diluent water,sedimentation treated water that is the third permeate to produce mixedwater. As shown in FIG. 11, it is possible to employ an arrangement inwhich sedimentation treated water is not filtered at the secondclarifier 232 to be designated as diluent water, the sea water 200A ismixed with, as diluent water, the sedimentation treated water to producemixed water, and the mixed water is filtered using the third clarifier210.

In the second embodiment, the first clarifier 222 is configured tofilter mixed water transferred to the first clarifier 222 by at leastone of a microfiltration membrane (MF membrane) and an ultrafiltrationmembrane (UF membrane), while it may be configured to filter the mixedwater by a sand filtration means having a sand filter. The secondembodiment is configured in this matter to produce an advantageouseffect in that impurities of a large amount of water can be removed withlow power.

In the arrangement in which sand filtration is carried out, the firstclarifier 222 may be configured to carry out sand filtration by onestage, or by two or more stages.

By the stage of sand filtration is meant the number of sand filtersconnected in tandem.

In the arrangement in which sand filtration is carried out, the firstclarifier 222 may be configured to further filter mixed water, which hasbeen subjected to sand filtration, by at least one of a microfiltrationmembrane (MF membrane) and an ultrafiltration membrane (UF membrane).

In the arrangement in which the first clarifier 222 is to carry out sandfiltration, the first clarifier 222 is provided with a cleaning means(not shown) for cleaning a sand filter layer.

In the second embodiment, the second clarifier 232 is configured tofilter sedimentation treated water, which has been transferred to thesecond clarifier 232, by at least one of a microfiltration membrane (MFmembrane) and an ultrafiltration membrane (UF membrane), while thesecond clarifier 232 may be configured such that sedimentation treatedwater is filtered by a sand filtration means.

In the arrangement of carrying out sand filtration, the second clarifier232 may be configured to carry out the sand filtration by one stage, orby two or more stages.

In the arrangement of carrying out sand filtration, the second clarifier232 may be configured such that sedimentation treated water, which hasbeen filtered by sand filtration means, is further filtered by least oneof a microfiltration membrane (MF membrane) and an ultrafiltrationmembrane (UF membrane).

In the arrangement in which the second clarifier 232 carries out sandfiltration, it is provided with a cleaning means (not shown) forcleaning a sand filter layer.

When the second embodiment includes the third clarifier 210, the thirdclarifier 210 is configured such that the sea water transferred to thethird clarifier 210 is filtered by at least one of a microfiltrationmembrane (MF membrane) and an ultrafiltration membrane (UF membrane) inthe second embodiment, while the third clarifier 210 may be configuredsuch that the sea water is filtered by a sand filtration means.

In the arrangement of carrying out sand filtration, the third clarifier210 may be configured to carry out the sand filtration by one stage, orby two or more stages.

In the arrangement of carrying out sand filtration, the third clarifier210 may be configured such that sea water, which has been subjected tosand filtration, is further filtered by least one of a microfiltrationmembrane (MF membrane) and an ultrafiltration membrane (UF membrane).

In the arrangement in which the third clarifier 210 carries out sandfiltration, it is provided with a cleaning means (not shown) forcleaning a sand filter layer.

In the second embodiment, it is possible to employ an arrangement inwhich power generation is made by utilizing natural energies (e.g., wavepower, tidal power, wind power, solar power and geothermal sources), andthe thus produced power from natural energies is utilized as a drivingpower for a pump or the like of the sea water desalinating apparatus ofthe second embodiment. The second embodiment is advantageous in that itis possible to suppress generation of gasses, such as carbon dioxide,which may be able to cause influences on the environment, hold inrunning out of fossil fuels, or prevent risks, such as nuclearaccidents, by utilizing power produced from natural energies.

The sea water desalinating apparatus 201 of the second embodiment isprovided with the water turbine 225 in the mixed water treatment part202, while a pressure converter (pressure recovery device) forconverting the pressure of the second concentrated water, which has beenpressure-fed from the first reverse osmosis membrane device 223, to thepressure for transferring mixed water directly (without intervention ofelectricity) to the first reverse osmosis membrane device 223, may beprovided in place of the water turbine 225.

When the pressure converter is provided, the sea water desalinatingapparatus 201 of the second embodiment is configured such that thesecond concentrated water, which has been pressure-fed from the firstreverse osmosis membrane device 223, is transferred to the pressureconverter, and the second concentrated water used at the pressureconverter is transferred to a concentrated water storage tank. The seawater desalinating apparatus 201 of the second embodiment is configuredsuch that mixed water is transferred to the pressure converter before itpasses through the first pump 224, and mixed water pressurized at thepressure converter is transferred to the first reverse osmosis membranedevice 223 via the first pump 224.

The sea water desalinating apparatus 201 of the second embodiment thusconfigured is advantageous in that the power of the first pump 224 canbe saved.

Still furthermore, in the second embodiment, before transferring theinorganic wastewater 200B to the sedimentation separation tank 231, theinorganic wastewater 200B may be adjusted in pH to close to neutral(e.g., pH 4 to pH 10) by an alkali (e.g., sodium hydroxide, etc.) or anacid (e.g., nitric acid, sulphuric acid, hydrochloric acid, etc.).Before transferring the inorganic wastewater 200B to the sedimentationseparation tank 231, the inorganic wastewater 200B may be subjected tooxidation or reduction by an oxidizing agent (e.g., hydrogen peroxide,sodium hypochlorite, etc.) or a reducing agent (e.g., sodium bisulfite,sodium thiosulfate, etc.).

In the second embodiment, sedimentation treated water is mixed, asdiluent water, into the sea water 200A, but the inorganic wastewater200B, which is not subjected to sedimentation, may be mixed, as diluentwater, into the sea water 200A. In the second embodiment, when inorganicwastewater, which is not subjected to sedimentation, is used as diluentwater, the inorganic wastewater 200B may be adjusted in pH to close toneutral (e.g., pH 4 to pH 10) before mixing the inorganic wastewaterinto the sea water 200A. The inorganic wastewater 200B may be subjectedto oxidation or reduction before mixing the inorganic wastewater 200Binto the sea water 200A.

According to the second embodiment, it is possible to efficientlyproduce purified water, such as fresh water 200C, while utilizing theinorganic wastewater 200B.

Third Embodiment

Now, the description will be made for a fresh water generating apparatusand a fresh water generating method, of the third embodiment.

Meanwhile, in a conventional sea water desalinating method, sea watermust be pressurized by a pump or the like and pressure fed to a reverseosmosis membrane device in order to carry out filtration of sea water bythe reverse osmosis membrane device, which poses a problem in that thehigher the salt concentration of sea water, the larger the energyrequired.

On the other hand, in addition to the above issue regarding sea water,wastewater containing organic matter represented by, for example, sewagewater (hereinafter referred also to as “organic wastewater”),biologically treated water produced by biologically treating organicwastewater, wastewater containing inorganic matter, such as heavy metal,represented by wastewater of a factory for manufacturing metal, such assteel (hereinafter referred also to as “inorganic wastewater”), orsedimentation treated wastewater produced by subjecting inorganicwastewater to sedimentation and separation is currently released to theoceans or rivers, which poses a problem in that most of wastewater isnot efficiently utilized.

The wastewater, treated wastewater or the like is low salt concentrationwastewater having a salt concentration lower than sea water, andtherefore when they are efficiently utilized as fresh water resources,it is assumed that the wastewater may be able to be desalinated by wayof reverse osmosis membrane filtration even with a relatively lowpressure pump. However, these low salt concentration wastewaters do notexhaustlessly exist unlike sea water, and therefore there may be a casein which a stabilized amount of fresh water may not be produced asresources of fresh water, or a stabilized amount of fresh water may notbe produced by way of filtration using a low pressure pump, since thesalt concentration may be greatly fluctuated depending on thecircumstances, which leads to a fear that a predetermined amount offresh water may not be able to be stably produced.

In consideration of the above problems, an object of the thirdembodiment is to provide a fresh water generating apparatus and a freshwater generating method that are capable of efficiently producing freshwater.

First, the description will be made for a fresh water generatingapparatus of the third embodiment.

FIG. 12 is a schematic block diagram of the fresh water generatingapparatus of the third embodiment.

As shown in FIG. 12, a fresh water generating apparatus 301 of the thirdembodiment includes a first treatment part 302 that separates low saltconcentration wastewater 300B having a salt concentration lower than seawater 300A into first permeate and first concentrated water by way ofreverse osmosis membrane filtration, and a second treatment part 303that mixes, as diluent water, the first concentrated water produced atthe first treatment part, into the sea water 300A to produce mixedwater, and separates the mixed water into second permeate and secondconcentrated water by way of reverse osmosis membrane filtration.

The fresh water generating apparatus 301 of the third embodiment isconfigured such that the low salt concentration wastewater 300B istransferred to the first treatment part 302, and the second concentratedwater is transferred, as concentrated water 300E, to a concentratedwater storage tank (not shown).

The fresh water generating apparatus 301 of the third embodiment isconfigured such that the first permeate is produced as fresh water 300Cand the second permeate is produced as fresh water 300D.

The sea water 300A is water containing salt, and for example, has a saltconcentration of about 1.0 to 8.0% by mass, and more specifically has asalt concentration of 2.5 to 6.0% by mass.

The sea water 300A is not herein necessarily limited to water in thesea, and is intended to include water in land area, such as water oflake (salt lake, brackish lake), water of swamps, and water of pond, aslong as they are water having a salt concentration of 1.0% by mass ormore.

The low salt concentration wastewater 300B is water having a saltconcentration lower than the sea water 300A. The low salt concentrationwastewater 300B is, for example, wastewater having a salt concentrationrelative to the sea water 300A of 1:not more than 0.1, and moregenerally 1:not more than 0.01.

The low salt concentration wastewater 300B is wastewater containingorganic matter (hereinafter referred also to as “organic wastewater”),wastewater containing inorganic matter (hereinafter referred also to as“inorganic wastewater”), or wastewater containing organic matter andinorganic matter.

The organic wastewater is wastewater having a BOD (Biochemical OxygenDemand), as an index of organic matter concentration, of 2000 mg/L orlower, and more specifically wastewater having a BOD of about 200 mg/L.Examples of the organic wastewater include sewage water (e.g., domesticwastewater or rainwater flowing into sewage pipes), and industrialwastewater (wastewater discharged from, for example, a food factory, achemical factory, a factory in electronics industry and a pulp plant).

The inorganic wastewater is wastewater containing inorganic matter andhaving a low concentration of organic matter, and, for example,wastewater having a BOD (Biochemical Oxygen Demand) of 50 mg/L or lower,and preferably wastewater having a BOD of 10 mg/L or lower. Examples ofthe inorganic wastewater include industrial wastewater (e.g., wastewaterdischarged from various factories, such as a steel factory, a chemicalfactory and a factory in electronics).

The low salt concentration wastewater 300B may be supernatant waterproduced by subjecting wastewater to sedimentation and separation in asedimentation separation tank, or permeate produced by way of filtrationand clarification by a microfiltration membrane (MF membrane), anultrafiltration membrane (UF membrane) or a sand filtration tank. Fororganic wastewater, the low salt concentration wastewater 300B may bebiologically treated water produced by purification of the organicwastewater with biological species.

By the clarifying is herein meant rougher filtration than reverseosmosis membrane filtration, that is, a treatment carried out prior tothe filtration by the reverse osmosis membrane device and made to removeimpurities (e.g., solid matter or the like) coarser than those filteredby a reverse osmosis membrane.

By the purification with biological species is herein meantdecomposition of organic matter contained in water with biologicalspecies, such as bacteria, protozoa and metazoan. A specific example ofsuch treatment includes aeration using activated sludge.

The reverse osmosis membrane as employed may be of a so-called hollowfiber membrane type that is formed into a hollow filament shape having adiameter of several millimeters and made of a material, such ascellulose acetate, aromatic polyamide and polyvinyl alcohol, a so-calledtubular membrane type that has a diameter of about several centimeters,greater than the hollow fiber membrane type, a so-called spiral membranetype that has an envelope shape kept wound into roll with a substratesuch as mesh disposed inside when in use, or of other conventionaltypes.

The first treatment part 302 includes a first reverse osmosis membraneunit 321 that separates the low salt concentration wastewater 300B intofirst permeate and first concentrated water by way of reverse osmosismembrane filtration, and is configured to pressure-feed the low saltconcentration wastewater 300B to the first reverse osmosis membrane unit321 via a first pump 322.

The first treatment part 302 includes a first salt concentrationmeasurement means 323 that measures the salt concentration of the lowsalt concentration wastewater 300B transferred to the first reverseosmosis membrane unit 321, and a first flow rate adjustment mechanism324 that adjusts the flow rate of the first permeate.

An example of the first salt concentration measurement means 323includes an instrument or device that is provided with an electricalconductivity meter or an ion counter for measuring the saltconcentration.

The first salt concentration measurement means 323 preferably has afunction of measuring the electric conductivity since the electricconductivity has a correlation relative to the salt concentration and iseasy to be measured.

In addition, the electrical conductivity meter is not expensive and iseasy to be maintained, such that the first salt concentrationmeasurement means 323 provided with the electrical conductivity meter iseffective in saving the costs of a fresh water generating apparatus andmaintenance costs.

The second treatment part 303 includes a mixing tank 336 that mixes thefirst concentrated water as diluent water into the sea water 300A toproduce mixed water, and a second reverse osmosis membrane unit 331 thatseparates the mixed water into second permeate and second concentratedwater by way of reverse osmosis membrane filtration, and is configuredto pressure-feed the mixed water to the second reverse osmosis membraneunit 331 via a second pump 332.

The fresh water generating apparatus 301 of the third embodiment isconfigured such that the sea water 300A is transferred into the mixingtank 336, and the first concentrated water as diluent water istransferred into the mixing tank 336.

The second treatment part 303 includes a second flow rate adjustmentmechanism 334 for adjusting the flow rate of the sea water 300A.

The first flow rate adjustment mechanism 324 and the second flow rateadjustment mechanism 334 are respectively provided with openingregulation valves, such as butterfly valves whose opening degrees arecapable of being adjusted based on signals transmitted from the firstsalt concentration measurement means 323 to change the flow rate of thefirst permeate and the flow rate of the sea water 300A, respectively.

The fresh water generating apparatus 301 of the third embodimentincludes a signal transmission mechanism 304 that transmits signals,which have been emitted from the first salt concentration measurementmeans 323, for example, as control signals for changing the openingdegrees of the opening regulation valves, to the first flow rateadjustment mechanism 324 and the second flow rate adjustment mechanism334, respectively.

The fresh water generating apparatus 301 of the third embodiment isconfigured such that the amount of the first permeate produced at thefirst treatment part 302, and the amount of the second permeate producedat the second treatment part 303 are controlled based on the measuredvalues obtained by the first salt concentration measurement means 323.Specifically, the fresh water generating apparatus 301 of the thirdembodiment is configured such that the amount of the first permeateproduced at the first treatment part 302 and the amount of the secondpermeate produced at the second treatment part 303 are controlledrespectively by the adjustment of the flow rate of the first permeate bythe first flow rate adjustment mechanism 324 and the adjustment of theamount of the sea water 300A by the second flow rate adjustmentmechanism 334, based on the measured values obtained by the first saltconcentration measurement means 323 and transmitted by the signaltransmission mechanism 304.

The fresh water generating apparatus 301 of the third embodiment isconfigured such that, when the measured value obtained by the first saltconcentration measurement means 323 is equal to or less than apredetermined reference value, control is made to increase the amount ofthe first permeate produced at the first treatment part 302, anddecrease the amount of the second permeate produced at the secondtreatment part 303.

Whilst the fresh water generating apparatus 301 of the third embodimentis configured in the manner described above, the description will behereinafter made for the fresh water generating method of the thirdembodiment.

The fresh water generating method of the third embodiment includescarrying out a first treatment step of separating the low saltconcentration wastewater 300B into the first permeate and the firstconcentrated water by the first reverse osmosis membrane unit 321, and asecond treatment step of mixing, as diluent water, the firstconcentrated water produced by the first treatment step into the seawater 300A within the mixing tank 336 to produce mixed water andseparating the mixed water into the second permeate and the secondconcentrated water by the second reverse osmosis membrane unit 331 toproduce permeate of each step as fresh water.

According to the fresh water generating method of the third embodiment,the salt concentration of the low salt concentration wastewater 300B ismeasured by the first salt concentration measurement means 323, and theamount of the permeate produced by the first treatment step and theamount of the permeate produced by the second treatment step arecontrolled, based on the measured value obtained by this measurement.

Specifically, according to the fresh water generating method of thethird embodiment, the amount of the first permeate produced at the firsttreatment part 302 and the amount of the second permeate produced at thesecond treatment part 303 are controlled respectively by the adjustmentof the flow rate of the first permeate by the first flow rate adjustmentmechanism 324 and the adjustment of the flow rate of the sea water 300Aby the second flow rate adjustment mechanism 334, based on the measuredvalues obtained by the first salt concentration measurement means 323and transmitted by the signal transmission mechanism 304.

According to the fresh water generating method of the third embodiment,control is made to increase the amount of the first permeate produced atthe first treatment part 302 and decrease the amount of the secondpermeate produced at the second treatment part 303, when the measuredvalue obtained by the first salt concentration measurement means 323 isequal to or less than a predetermined reference value.

Although no detailed description will be made herein, it is possible toapply various devices or instruments used in a conventional fresh watergenerating apparatus to the fresh water generating apparatus of thepresent invention to such an extent not to deteriorate the advantageouseffects of the present invention. Also, it is possible to employ variouscontrol or operating method of facilities of a conventional fresh watergenerating method in the fresh water generating method of the presentinvention.

For example, although the fresh water generating apparatus 301 of thethird embodiment includes the first flow rate adjustment mechanism 324,the fresh water generating apparatus of the present invention may beprovided with a first inverter 325 that changes the rotational rate ofthe first pump 322 based on the measured result of the first saltconcentration measurement means 323, and may have the signaltransmission mechanism 304 connected to the first inverter 325, as shownin FIG. 13.

Although the fresh water generating apparatus 301 of the thirdembodiment includes the second flow rate adjustment mechanism 334, thefresh water generating apparatus of the present invention may beprovided with a second inverter 335 that changes the rotational rate ofthe second pump 332 based on the measured result of the first saltconcentration measurement means 323, and may have the signaltransmission mechanism 304 connected to the second inverter 335, asshown in FIG. 13.

According to the fresh water generating apparatus 301 of the thirdembodiment, the second treatment part 303 may be provided with a secondsalt concentration measurement means 333 that measures the saltconcentration of the sea water 300A transferred to the mixing tank 336,as shown in FIG. 14.

Herein, when the salt concentration of the sea water 300A has beenfluctuated, the generation efficiency of the second permeate at thesecond reverse osmosis membrane unit 331 is also fluctuated.Specifically, when the salt concentration of the sea water 300A has beenlowered, the generation efficiency of the second permeate is increased,and when the salt concentration of the sea water 300A has beenincreased, the generation efficiency of the second permeate is lowered.

In order to deal with this, according to the fresh water generatingapparatus 301 of the third embodiment, control is made such that theflow rate of the first permeate is adjusted by the first flow rateadjustment mechanism 324 and the flow rate of the sea water 300A isadjusted by the second flow rate adjustment mechanism 334, based on themeasured value obtained by the first salt concentration measurementmeans 323 and transmitted by the signal transmission mechanism 304, inwhich the flow rate of the sea water 300A is corrected based on thegeneration efficiency of the second permeate determined according to thesalt concentration of the sea water 300A produced by the second saltconcentration measurement means 333.

Thus, according to the third embodiment, fresh water can be efficientlyand securely produced.

Fourth Embodiment

Now, the description will be made for a fresh water generating apparatusand a fresh water generating method, of the fourth embodiment.

Meanwhile, in a conventional sea water desalinating technique, sea watermust be pressurized and pressure fed to a reverse osmosis membrane unitby a pump or the like in order to carry out filtration of sea water bythe reverse osmosis membrane unit, which poses a problem in that thehigher the salt concentration of sea water, the larger the energyrequired.

On the other hand, in addition to the above issue regarding sea water,wastewater containing organic matter represented by, for example, sewagewater, biologically treated wastewater produced by biologically treatingorganic wastewater, inorganic wastewater containing inorganic matter,such as metal, represented by, for example, wastewater from a factoryfor manufacturing metal, such as steel, or sedimentation treatedwastewater produced by subjecting inorganic wastewater to pretreatment,such as pH adjustment to solidify the same, and subjecting anintermediate to sedimentation treatment, is currently released to theoceans or rivers, which poses a problem in that most of them are noteffectively utilized.

These wastewater or treated wastewater are low salt concentrationwastewater having a salt concentration lower than sea water, andtherefore when they are efficiently utilized as fresh water resources,it is assumed that the wastewater may be able to be desalinated by wayof reverse osmosis membrane filtration even with a relatively lowpressure pump.

Meanwhile, the amount of intake of these low salt concentrationwastewaters is greatly fluctuated depending on the circumstances. Forexample, for sewage water, the amount of intake is fluctuated dependingon the time or season, and for industrial wastewater, the amount ofintake is fluctuated depending on the amount of production, orproduction process.

That is, these low salt concentration wastewaters do not exhaustlesslyexist unlike sea water, and therefore there may be a case in which arequired amount may not be stably produced as fresh water resources, ora case in which they must be appropriately disposed of when the amountof intake is large even in an arrangement with a storage tank.

Accordingly, there may cause problems in that a predetermined amount offresh water cannot be stably produced, or fresh water resources capableof producing fresh water at low cost cannot be sufficiently utilized,thus causing deterioration in efficiency.

In order to deal with these problems, it is assumed to take a measurewhich uses a device provided with an excessively large storage tank, butit requires a huge space for installation.

In consideration of the above problems, an object of the fourthembodiment is to provide a fresh water generating apparatus and a freshwater generating method that are capable of efficiently producing apredetermined amount of fresh water in a stabilized manner, whileomitting the necessity to provide a huge space for an excessively largestorage tank.

First, the description will be made for a fresh water generatingapparatus of the fourth embodiment.

FIG. 15 is a schematic block diagram of the fresh water generatingapparatus of the fourth embodiment.

As shown in FIG. 15, a fresh water generating apparatus 401 of thefourth embodiment includes a first treatment part 402 that separates lowsalt concentration wastewater 400B having a salt concentration lowerthan sea water 400A into first permeate and first concentrated water byway of reverse osmosis membrane filtration, and a second treatment part403 that mixes, as diluent water, the first concentrated water producedat the first treatment part into the sea water 400A to produce mixedwater, and separates the mixed water into second permeate and secondconcentrated water by way of reverse osmosis membrane filtration.

The fresh water generating apparatus 401 of the fourth embodiment isconfigured such that the low salt concentration wastewater 400B is fedto the first treatment part 402, and the second concentrated water istransferred, as concentrated water 400E, to a concentrated water storagetank (not shown).

The fresh water generating apparatus 401 of the fourth embodiment isconfigured such that the first permeate is produced as fresh water 400Cand the second permeate is produced as fresh water 400D.

The sea water 400A is water containing salt, and for example, has a saltconcentration of about 1.0 to 8.0% by mass, and generally has a saltconcentration of 2.5 to 6.0% by mass.

The sea water 400A is not herein necessarily limited to water in thesea, and is intended to include water in land area, such as water oflake (salt lake, brackish lake), water of swamps, and water of pond, aslong as they are water having a salt concentration of 1.0% by mass ormore.

The low salt concentration wastewater 400B is water having a saltconcentration lower than the sea water 400A. The low salt concentrationwastewater 400B is, for example, wastewater having a salt concentrationrelative to the sea water 400A of 1:not more than 0.1, and moregenerally 1:not more than 0.01.

Examples of the low salt concentration wastewater 400B include organicwastewater containing organic matter and inorganic wastewater containinginorganic matter.

The organic wastewater is, for example, wastewater having a BOD(Biochemical Oxygen Demand), as an index of organic matterconcentration, of 2000 mg/L or lower, and more generally wastewaterhaving a BOD of about 200 mg/L. Examples of the organic wastewaterinclude sewage water (e.g., domestic wastewater or rainwater flowinginto sewage pipes), and industrial wastewater (wastewater dischargedfrom, for example, a food factory, a chemical factory, a factory inelectronics industry and a pulp plant).

The inorganic wastewater is, for example, wastewater having a lowconcentration of organic matter, and wastewater having a BOD(Biochemical Oxygen Demand) of 50 mg/L or lower, and preferablywastewater having a BOD of 10 mg/L or lower. Examples of the inorganicwastewater include industrial wastewater (e.g., wastewater dischargedfrom various factories, such as a steel factory, a chemical factory anda factory in electronics).

The low salt concentration wastewater 400B may be supernatant waterproduced by subjecting wastewater (organic wastewater or inorganicwastewater) to sedimentation and separation in a sedimentationseparation tank, or permeate produced by way of filtration andclarification by a microfiltration membrane (MF membrane), anultrafiltration membrane (UF membrane), or a sand filtration tank. Fororganic wastewater, the low salt concentration wastewater 400B may bebiologically treated water produced by purification of the organicwastewater with biological species.

By the clarifying is herein meant rougher filtration than reverseosmosis membrane filtration, that is, a treatment carried out prior tothe filtration by the reverse osmosis membrane device and made to removeimpurities (e.g., solid matter or the like) coarser than those filteredby a reverse osmosis membrane.

By the purification with biological species is herein meantdecomposition of organic matter contained in water with biologicalspecies, such as bacteria, protozoa and metazoan. A specific example ofsuch treatment includes aeration using activated sludge.

The reverse osmosis membrane as employed may be of a so-called hollowfiber membrane type, a so-called tubular membrane type, a so-calledspiral membrane type, or of other conventional types.

The first treatment part 402 includes a plurality of first reverseosmosis membrane units 421 that separate the low salt concentrationwastewater 400B into first permeate and first concentrated water by wayof reverse osmosis membrane filtration, a plurality of first pumps 422that pressure-feed the low concentration wastewater 400B respectively tothe first reverse osmosis membrane units 421, and a first flow ratemeasurement device 423 as a flow rate measurement means that measuresthe flow rate of the low salt concentration wastewater 400B to be fed tothe first treatment part 402.

The second treatment part 403 includes a mixing tank 436 that mixes thefirst concentrated water as diluent water into the sea water 400A toproduce mixed water, a plurality of second reverse osmosis membraneunits 431 that separate the mixed water into second permeate and secondconcentrated water by way of reverse osmosis membrane filtration, and aplurality of second pumps 432 that pressure-feed the mixed waterrespectively to the second reverse osmosis membrane units 431, and isconfigured to pressure-feed the mixed water to the second reverseosmosis membrane units 431 via the second pumps 432.

The fresh water generating apparatus 401 of the fourth embodiment isconfigured such that the sea water 400A is fed into the mixing tank 436with a pump (not shown), and the first concentrated water as diluentwater is transferred into the mixing tank 436.

The second treatment part 403 includes a second flow rate measurementdevice 435 for measuring the flow rate of the sea water 400A, and acontrol valve as a flow rate adjustment mechanism 434 for adjusting theflow rate of the sea water 400A to be fed to the mixing tank 436.

The fresh water generating apparatus 401 of the fourth embodiment isconfigured such that the amount to be filtered by each of the firsttreatment part 402 and the second treatment part 403 is controlled basedon the measured value of the flow rate measured by the first flow ratemeasurement device 423.

Specifically, control is made such that, on the basis of the increase inmeasured value, the number of the first reverse osmosis membrane units421 to be operated at the first treatment part 402 is increased, whilethe number of the second reverse osmosis membrane units 431 to beoperated at the second treatment part 403 is decreased.

Giving further description on this point, the first flow ratemeasurement device 423 is electrically connected to each of the firstpumps 422, and control is made such that the required number of thefirst pumps 422 is operated based on the measured value by the firstflow rate measurement device 423. Thus, as the measured value isincreased, the number of the first pumps 422 to be operated isincreased, while, corresponding thereto, the number of the first reverseosmosis membrane units 421 to perform reverse osmosis membranefiltration is increased such that the amount to be treated at the firsttreatment part 402 is increased.

The first flow rate measurement device 423 is electrically connected tothe flow rate adjustment mechanism 434, while the second flow ratemeasurement device 435 is electrically connected to each of the secondpumps 432, such that the flow rate of the sea water 400A to be flowninto the mixing tank 436 of the second treatment part 403 is controlledbased on the measured value by the first flow rate measurement device423. Thus, as the measured value is increased, the flow rate of the seawater 400A at the second treatment part 403 is, on the contrary,decreased. Corresponding thereto, the measured value by the second flowrate measurement device 435 is lowered, such that the number of thesecond pumps 432 to be operated (that is, the number of the secondreverse osmosis membrane units 431 to be operated) based on thismeasured value (specifically, in consideration of the measured value andthe flow rate of the first concentrated water) is decreased.

It is a matter of course that, when the measured value has been lowered,control is made such that the number of the first reverse osmosismembrane units 421 to be operated at the first treatment part 402 isdecreased, while the number of the second reverse osmosis membrane units431 to be operated at the second treatment part 403 is increased.

Furthermore, in the fourth embodiment, there is provided a bypass line440 that allows a part of the low salt concentration wastewater 400B atthe first treatment part 402 to be bypassed to the mixing tank 436, suchthat the bypass-fed amount can be controlled based on the measured valueby the first flow rate measurement device 423.

For example, when the flow rate, which exceeds beyond the reverseosmosis membrane filtration capacity at the first treatment part 402,has been measured, control is made to feed an amount of the wastewaterequivalent to the exceeding amount to the mixing tank 436 via the bypassline 440.

In the fourth embodiment, the thus provided bypass line 440 enables thelow salt concentration wastewater 400B to be utilized as fresh waterresources without disposing it, even when an unexpected amount of thelow salt concentration wastewater 400B has been measured.

In the fourth embodiment, when the flow rate which exceeds beyond thefiltration capacity at the first treatment part 402 has been measured,control is made to feed such exceeded amount of the wastewater to themixing tank 436 via the bypass line 440, but the present invention isnot necessarily limited thereto. For example, control may be made suchthat, when the measured value by the first flow rate measurement device423 has exceeded a predetermined value, a part of the low saltconcentration wastewater 400B at the first treatment part 402 isbypassed to the mixing tank 436.

Whilst the fresh water generating apparatus 401 of the fourth embodimentis configured in the manner described above, the description will behereinafter made for the fresh water generating method of the fourthembodiment.

The fresh water generating method of the fourth embodiment includescarrying out a first treatment step of separating the low saltconcentration wastewater 400B having a salt concentration lower than thesea water 400A into first permeate and first concentrated water by wayof filtration at the first reverse osmosis membrane units 421, and asecond treatment step of feeding the first concentrated water producedby the first treatment step, as diluent water, to the mixing tank 436 tobe mixed into the sea water 400A at the mixing tank 436 to produce mixedwater, and separating the mixed water into second permeate and secondconcentrated water by way of filtration at the second reverse osmosismembrane units 431 to produce, as fresh water, the first permeate andthe second permeate separated at the respective steps.

In the fourth embodiment, the flow rate of the low salt concentrationwastewater 400B at the first flow rate measurement device 423 ismeasured, and the amount of wastewater to be filtered at each of thefirst treatment part 402 and the second treatment part 403 is controlledbased on the measured value.

Specifically, the number of the first pumps 422 (the number of the firstreverse osmosis membrane units 421) to be operated at the firsttreatment part 402 and the flow rate adjustment mechanism 434 at thesecond treatment part 403 are controlled to resultingly control thenumber of the second pumps 432 (the number of the second reverse osmosismembrane units 431), thereby producing fresh water.

In the fresh water generating method of the fourth embodiment, it ispossible to employ an arrangement, in which the flow rate of the lowsalt concentration wastewater 400B is measured by the first flow ratemeasurement device, and the flow rate of the low salt concentrationwastewater 400B to be fed to the mixing tank 436 via the bypass line 440is controlled based on the measured value. In this arrangement, when theflow rate exceeding the reverse osmosis membrane filtration capacity atthe first treatment part 402 has been measured, control is preferablymade such that the exceeding amount of the low salt concentrationwastewater 400B is fed to the mixing tank 436 via the bypass line 440.However, the present invention is not necessarily limited to this, andcontrol may be made such that, when a flow rate exceeding apredetermined flow rate value has been measured at the first flow ratemeasuring device 423, the excessive amount of the wastewater is fed tothe mixing tank 436 via the bypass line 440.

The fresh water generating apparatus 401 and the fresh water generatingmethod, of the present invention are not necessarily limited to those ofthe fourth embodiment described above, and they may be appropriatelymodified within an intended scope of the present invention.

For example, although no illustration is made, it is possible to employan arrangement, in which the bypass line 440 is provided at a flow rateregulating valve, by which the feeding amount to the mixing tank 436 viathe bypass line 440 is controlled.

The present invention is not necessarily limited to the arrangement, inwhich the number of the first pumps 422 to be operated, the number ofthe second pumps 432 to be operated, or the like are controlled based ononly the measured value by the first flow rate measurement device 423,and it is possible to employ an arrangement, in which the number of thefirst pumps 422 to be operated, the number of the second pumps 432 to beoperated, etc., are controlled, also taking into account the measuredvalue by a flow rate measurement device disposed at a different place.

For example, it is possible to employ an arrangement, in which a flowrate measurement device for measuring the flow rate of firstconcentrated water is disposed at a downstream side of the first reverseosmosis membrane units 421, and the number of the second pumps 432 to beoperated and the amount of the low salt concentration wastewater 400B tobe fed to the mixing tank 436 via the bypass line 440 are adjusted andcontrolled based on both the measured value by this flow ratemeasurement device and the measured value by the first flow ratemeasurement device 423.

In the fourth embodiment, the number of the second pumps 432 to beoperated is controlled based on the measured value by the second flowrate measurement device 435 (specifically, taking into account themeasured value and the flow rate of the first concentrated water).However, taking into account that the low salt concentration wastewater400B is sometimes fed to the mixing tank 436 via the bypass line 440, itis possible to employ an arrangement, in which a flow rate measurementdevice for measuring the flow rate of the first concentrated water andthe feeding amount by the bypass line is provided, such that the numberof the second pumps 432 to be operated is controlled based on the totalvalue of the measure value of the second flow rate measurement device435, the measured value of the flow rate of the first concentrated waterand the measured value of the bypass-fed amount. It is also possible toemploy an arrangement, in which a flow rate measurement device formeasuring the flow rate of the first concentrated water is not providedsuch that a value determined by calculation according to the amount fedto the first reverse osmosis membrane units 421 is utilized.

Thus, according to the fourth embodiment, it is possible to efficientlyand stably produce a predetermined amount of fresh water, while omittingthe necessity to provide a huge space for an excessively large storagetank.

EXAMPLES

Now, a more specific description will be made for the present inventionby citing examples and comparative examples.

First, a specific description will be made for the first embodiment.

Test Example 1

As shown in FIG. 16, biologically treated water, that is, diluent waterG produced by biologically treating sewage water as organic wastewaterB, and sea water A are mixed together in the amounts indicated in Table1 to produce mixed water, and the mixed water produced by the mixing isfed to the first reverse osmosis membrane unit 23 via the first pump 24to be filtered. Thus, fresh water C that is permeate, and concentratedwater D are produced. The feed pressure (ata) of the mixed water fromthe first pump 24 to the first reverse osmosis membrane unit 23, thepower consumption of the first pump 24 (W), and the amount (L) of thefresh water C that is the permeate and the concentrated water D, duringfiltration, are determined by calculation. The results of thecalculation are shown in Table 1 and FIG. 17.

By the unit power ratio in Table 1 is meant a ratio of power consumedfor filtration per unit amount of each mixed water flow through, whenthe power consumed for filtration per unit amount of sea water A notdiluted with biologically treated water flow through is 100. By thesymbol “%” as a unit of the salt concentration of mixed water is meant“% by mass”.

TABLE 1 Amount of Salt Power biologically concentration Amount ofRecovery consumption Unit Amount of treated Amount of of mixed Amount ofconcentrated rate of Power per unit amount Power sea water water mixedwater water Pressure permeate water permeate consumption of permeateRatio (L) (L) (L) (% by mass) (ata) (L) (L) (%) (W) (W/L) (%) 100 0 1003.50 50.0 40 60 40.0 100 2.50 100.0 100 10 110 3.18 47.3 48 62 43.6 1042.17 86.7 100 20 120 2.92 45.0 56 64 46.7 108 1.93 77.1 100 30 130 2.6943.1 64 66 49.2 112 1.75 70.0 100 40 140 2.50 41.4 72 68 51.4 116 1.6164.4 100 50 150 2.33 40.0 80 70 53.3 120 1.50 60.0 100 60 160 2.19 38.888 72 55.0 124 1.41 56.4 100 70 170 2.06 37.6 96 74 56.5 128 1.33 53.3100 80 180 1.94 36.7 104 76 57.8 132 1.27 50.8 100 90 190 1.84 35.8 11278 58.9 136 1.21 48.6 100 100 200 1.75 35.0 120 80 60.0 140 1.17 46.7

As shown in Table 1 or FIG. 17, as the sea water is further diluted withthe biologically treated water, the unit power ratio can be lowered.Also, with sea water of 1:diluent water of not less than 0.1, it isfound that the effect of reducing the power consumption can be produced.

Test Example 2 Example 1

In Example 1, the sea water A (salt concentration: 3.5% by mass) isdesalinated by using biologically treated water produced by biologicallytreating the sewage water in the manner mentioned below, by using a seawater desalinating device shown in FIG. 18.

First, sewage water as organic wastewater B is transferred to thebiological treatment part 3 at a flow rate of 100,000 ton/day, then thesewage water is biologically treated within the second biologicaltreatment tank 31 of the biological treatment part 3 to producebiologically treated water, then the biologically treated water isfiltered by using the second clarifier 32 that has a microfiltrationmembrane and is a submerged membrane to produce permeate, and then thepermeate is transferred via the second pump 34 to the second reverseosmosis membrane device 33, at which purified water E that is permeateand biologically treated water that is concentrated water are producedby using the second reverse osmosis membrane device 33. The purifiedwater E was produced at a flow rate of 70,000 ton/day, and thebiologically treated water that is the concentrated water was producedat a flow rate of 30,000 ton/day.

Then, the purified water E is recovered, and the biologically treatedwater that is the concentrated water is transferred, as diluent water,to the mixed water treatment part 2.

Then, the sea water A is transferred to the mixed water treatment part 2at a flow rate of 30,000 ton/day, then the biologically treated waterthat is the concentrated water is mixed, as diluent water, into the seawater A to produce mixed water (salt concentration: 1.8% by mass), thenthe mixed water is transferred via the first pump 24 to the firstreverse osmosis membrane device 23, by which fresh water C that ispermeate and concentrated water D are produced. Purified water that isthe fresh water C was produced at a flow rate of 36,000 ton/day, and theconcentrated water D was produced at a flow rate of 24,000 ton/day.

Accordingly, purified water (including the fresh water C) was producedat a flow rate of 106,000 ton/day.

Comparative Example 1

In Comparative Example 1, sea water A (salt concentration: 3.5% by mass)is desalinated by using a sea water desalinating apparatus shown in FIG.19 in the manner mentioned below.

First, sewage water as organic wastewater B is transferred to abiological treatment tank 7 at a flow rate of 100,000 ton/day, and thesewage water is biologically treated within the biological treatmenttank 7 to produce biologically treated water H. This biologicallytreated water is released to the outside.

Then, sea water A is transferred at a flow rate of 250,000 ton/day via afirst pump 8 to a reverse osmosis membrane device 9, by which freshwater I that is permeate and concentrated water J are produced. Purifiedwater that is the fresh water I was produced at a flow rate of 100,000ton/day, and the concentrated water was produced at a flow rate of150,000 ton/day.

The results of power consumed by the sea water desalinating methods ofExample 1 and Comparative Example 1, the amounts of purified waterproduced by the respective methods, and the like are shown in Table 2.

The amount of purified water produced is an amount including the amountof fresh water. Power consumed for driving the first pump and the secondpump is designated as the total power consumption (since the second pumpis not used in Comparative Example 1, power consumed for driving thefirst pump only is designated as the total power consumption). Annualpower consumption is calculated with the operating time per year being330×24 hours. Annual CO₂ emission is calculated with the CO₂ emissionper unit output being 0.41 kg-CO₂/kWh.

TABLE 2 Comparative Unit Example 1 Example 1 Amount of purified Ton/day106,000 100,000 water produced Power consumption kW 4,723 39,356 of 1stpump Power consumption kW 2,249 — of 2nd pump Total power kW 6.97239,356 consumption Annual power kWh/year 55,218,240 311,699,520consumption Annual CO₂ Ton/year 22,639 127,797 emission

The amount of purified water produced by the sea water desalinatingmethod of Example 1, which is within the range of the present invention,is substantially equal to the amount of purified water produced by thesea water desalinating method of Comparative Example 1, in which the seawater is desalinated without performing dilution. Regardless of thisfact, the total power consumption of Example 1 is significantly low ascompared with that of Comparative Example 1. Also, the annual CO₂emission of Example 1 is significantly low as compared with that ofComparative Example 1.

Now, a specific description will be made for the second embodiment.

Test Example 3

As shown in FIG. 20, diluent water 200G that is steel wastewater asinorganic wastewater, and sea water 200A are mixed together in theamounts shown in Table 3 to produce mixed water, then the mixed waterproduced by the mixing is fed to the first reverse osmosis membranedevice 223 via the first pump 224 by which the mixed water is filteredto produce fresh water 200C that is permeate and concentrated water200D. The feed pressure (MPa) of the mixed water from the first pump 224to the first reverse osmosis membrane device 223, the power consumption(W) of the first pump 224, and the amount (L) of the fresh water 200Cthat is permeate and the concentrated water 200D in filtration processare determined by calculation. The results of the calculation are shownin Table 3 and FIG. 21.

By the unit power ratio in Table 3 is meant a ratio of power consumedfor filtration per unit amount of each mixed water flow through, whenthe power consumed for filtration per unit amount of sea water A notdiluted with inorganic wastewater flow through is 100. By the symbol “%”as a unit of the salt concentration of mixed water is meant “% by mass”.

TABLE 3 Salt Power Amount of concentration Amount of Recoveryconsumption Unit Amount of inorganic Amount of of mixed Amount ofconcentrated rate of Power per unit amount Power sea water wastewatermixed water water Pressure permeate water permeate consumption ofpermeate Ratio (L) (L) (L) (% by mass) (MPa) (L) (L) (%) (W) (W/L) (%)100 0 100 3.50 5.00 40 60 40.0 100 2.50 100.0 100 10 110 3.21 4.75 48 6243.3 105 2.20 87.9 100 20 120 2.98 4.55 55 65 46.0 109 1.98 79.1 100 30130 2.77 4.38 63 67 48.3 114 1.81 72.5 100 40 140 2.60 4.23 70 70 50.3118 1.68 67.3 100 50 150 2.45 4.10 78 72 52.0 123 1.58 63.1 100 60 1602.32 3.99 86 74 53.5 128 1.49 59.6 100 70 170 2.20 3.89 93 77 54.8 1321.42 56.7 100 80 180 2.10 3.80 101 79 56.0 137 1.36 54.3 100 90 190 2.013.72 108 82 57.1 141 1.30 52.2 100 100 200 1.93 3.65 116 84 58.0 1461.26 50.3 100 110 210 1.85 3.59 124 86 58.9 151 1.22 48.7 100 120 2201.78 3.53 131 89 59.6 155 1.18 47.3 100 125 225 1.75 3.50 135 90 60.0158 1.17 46.7

As shown in Table 3 and FIG. 21, as the sea water is further dilutedwith diluent water, the unit power ratio can be lowered. Also, with seawater of 1:diluent water of not less than 0.1, it is found that theeffect of reducing the power consumption can be produced.

Test Example 4 Example 2

In Example 2, the sea water 200A (salt concentration: 3.5% by mass) isdesalinated by using sedimentation treated water that is supernatantwater produced by subjecting steel wastewater to aggregation andsedimentation in a sea water desalinating apparatus shown in FIG. 22 inthe manner mentioned below.

First, steel wastewater as inorganic wastewater 200B is transferred tothe sedimentation treatment part 203 at a flow rate of 100,000 ton/day,then the steel wastewater is subjected to sedimentation and separationwithin the sedimentation separation tank 231 of the sedimentationtreatment part 203 to produce sedimentation treated water that issupernatant water, then the sedimentation treated water is transferredto the second clarifier 232 that has a microfiltration membrane to befiltered to produce permeate, and then the permeate is transferred viathe second pump 234 to the second reverse osmosis membrane device 233,by which purified water 200E that is permeate and sedimentation treatedwater that is concentrated water are produced. The purified water 200Ewas produced at a flow rate of 70,000 ton/day, and the sedimentationtreated water that is the concentrated water was produced at a flow rateof 30,000 ton/day.

Then, the purified water 200E is recovered, and the sedimentationtreated water that is the concentrated water is transferred, as diluentwater, to the mixed water treatment part 202.

Then, the sea water 200A is transferred to the mixed water treatmentpart 202 at a flow rate of 30,000 ton/day, then the sedimentationtreated water that is the concentrated water is mixed, as diluent water,into the sea water 200A to produce mixed water (salt concentration:1.93% by mass), then the mixed water is transferred via the first pump224 to the first reverse osmosis membrane device 223, by which freshwater 200C that is permeate and concentrated water D are produced.Purified water that is the fresh water 200C was produced at a flow rateof 34,800 ton/day, and the concentrated water 200D was produced at aflow rate of 25,200 ton/day.

Accordingly, purified water (including the fresh water 200C) wasproduced at a flow rate of 104,800 ton/day.

Comparative Example 2

In Comparative Example 2, the sea water 200A (salt concentration: 3.5%by mass) is desalinated by using a sea water desalinating apparatusshown in FIG. 23 in the manner mentioned below.

First, steel wastewater as inorganic wastewater 200B is transferred tothe sedimentation treatment part 207 at a flow rate of 100,000 ton/day,then the steel wastewater is subjected to sedimentation and separationwithin the sedimentation separation tank 207 to produce sedimentationtreated water 200H that is supernatant water. This sedimentation treatedwater 200H is released to the outside.

The sea water 200A is transferred at a flow rate of 250,000 ton/day viathe first pump 208 to the reverse osmosis membrane device 209, by whichfresh water 200I that is permeate and concentrated water 200J areproduced. Purified water that is the fresh water 200I was produced at aflow rate of 100,000 ton/day, and the concentrated water was produced ata flow rate of 150,000 ton/day.

The results of power consumed by the sea water desalinating methods ofExample 2 and Comparative Example 2, the amounts of purified waterproduced by the respective methods, and the like are shown in Table 4.

The amount of purified water produced is an amount including the amountof fresh water. Power consumed for driving the first pump and the secondpump is designated as the total power consumption (since the second pumpis not used in Comparative Example 2, power consumed for driving thefirst pump only is designated as the total power consumption). Annualpower consumption is calculated with the operating time per year being330×24 hours. Annual CO₂ emission is calculated with the CO₂ emissionper unit output being 0.41 kg-CO₂/kWh.

TABLE 4 Comparative Unit Example 2 Example 2 Amount of purified Ton/day104,800 100,000 water produced Power consumption kW 4,925 39,356 of 1stpump Power consumption kW 2,249 — of 2nd pump Total power kW 7,17439,356 consumption Annual power kWh/year 56,817,728 311,696,000consumption Annual CO₂ Ton/year 23,295 127,795 emission

The amount of purified water produced by the sea water desalinatingmethod of Example 2, which is within the range of the present invention,is substantially equal to the amount of purified water produced by thesea water desalinating method of Comparative Example 2. Regardless ofthis fact, the total power consumption of Example 2 is significantly lowas compared with that of Comparative Example 2, in which the sea wateris desalinated without performing dilution. Also, the annual CO₂emission of Example 2 is significantly low as compared with that ofComparative Example 2.

DESCRIPTION OF THE REFERENCE NUMERALS

1: sea water desalinating apparatus, 2: mixed water treatment part, 3:biological treatment part, 4: methane fermentation part, 5:concentration difference power production part, 7: biological treatmenttank, 8: first pump, 9: reverse osmosis membrane device, 10: thirdclarifier, 21: first biological treatment tank, 22: first clarifier, 23:first reverse osmosis membrane device, 24: first pump, 25: waterturbine, 31: second biological treatment tank, 32: second clarifier, 33:second reverse osmosis membrane device, 34: second pump, 35: carrier, 35a: trapping member, 35 b: supporting member, 36: aeration means, A: seawater, B: organic wastewater, C: fresh water, D: concentrated water, E:purified water, F: industrial water, G: diluent water, H: biologicallytreated water, I: fresh water, J: concentrated water, 201: sea waterdesalinating apparatus, 202: mixed water treatment part, 203:sedimentation treatment part, 205: concentration difference powerproduction part, 207: sedimentation treatment part, 208: first pump,209: reverse osmosis membrane device, 210: third clarifier, 222: firstclarifier, 223: first reverse osmosis membrane device, 224: first pump,225: water turbine, 231: sedimentation separation tank, 232: secondclarifier, 233: second reverse osmosis membrane device, 234: secondpump, 200A: sea water, 200B: inorganic wastewater, 200C: fresh water,200D: concentrated water, 200E: purified water, 200F: industrial water,200G: diluent water, 200H: sedimentation treated water, 200I: freshwater, 200J: concentrated water, 301: fresh water generating apparatus,302: first treatment part, 303: second treatment part, 304: signaltransmission mechanism, 321: first reverse osmosis membrane unit, 322:first pump, 323: first salt concentration measurement means, 324: firstflow rate adjustment mechanism, 325: first inverter, 331: second reverseosmosis membrane unit, 332: second pump, 333: second salt concentrationmeasurement means, 334: second flow rate adjustment mechanism, 335:second inverter, 336: mixing tank, 300A: sea water, 300B: low saltconcentration wastewater, 300C: fresh water, 300D: fresh water, 300E:concentrated water, 401: fresh water generating apparatus, 402: firsttreatment part, 403: second treatment part, 421: first reverse osmosismembrane unit, 422: first pump, 423: first flow rate measurement device,431: second reverse osmosis membrane unit, 432: second pump, 434: flowrate adjustment mechanism, 435: second flow rate measurement device,436: mixing tank, 440: bypass line, 400A: sea water, 400B: low saltconcentration wastewater, 400C: fresh water, 400D: fresh water, 400E:concentrated water.

1. A fresh water generating method for generating fresh water by way ofreverse osmosis membrane filtration, comprising mixing sea water withlow salt concentration water having a salt concentration lower than seawater to produce mixed water, and subjecting the mixed water prepared bythe mixing to reverse osmosis membrane filtration, thereby generatingfresh water.
 2. A fresh water generating apparatus for generating freshwater by way of reverse osmosis membrane filtration, wherein the freshwater generating apparatus is configured such that sea water is mixedwith low salt concentration water having a salt concentration lower thansea water to produce mixed water, and the mixed water produced by themixing is subjected to reverse osmosis membrane filtration, therebygenerating fresh water.
 3. A sea water desalinating method fordesalinating sea water by way of filtration using a reverse osmosismembrane device, comprising carrying out a mixing step of mixing, asdiluent water, biologically treated water produced by biologicallytreating organic wastewater into sea water having a salt concentrationof 1.0 to 8.0% by mass to produce mixed water, and a mixed waterprocessing step of feeding the mixed water produced by the mixing stepto the reverse osmosis membrane device, at which the mixed water isfiltered, thereby desalinating the sea water.
 4. The sea waterdesalinating method according to claim 3, further comprising carryingout a wastewater treatment step of producing biologically treated waterby biologically treating organic wastewater, producing permeate byfiltering the biologically treated water by using a clarifier that hasat least one of a microfiltration membrane, an ultrafiltration membraneand a sand filtration means, and producing permeate that is purifiedwater and concentrated water by filtering the permeate using a reverseosmosis membrane device, wherein the biologically treated water used asthe diluent water in the mixing step is the concentrated water.
 5. Thesea water desalinating method according to claim 4, wherein filtrationwith the clarifier installed as a submerged membrane below the liquidlevel of a biological treatment tank for the biological treatment iscarried out in the wastewater treatment step.
 6. The sea waterdesalinating method according to claim 3, wherein the mixed water isfiltered using a clarifier that has at least one of a microfiltrationmembrane, an ultrafiltration membrane and a sand filtration means, priorto the filtration using the reverse osmosis membrane device, in themixed water treatment step.
 7. The sea water desalinating methodaccording to claim 6, wherein the mixed water is biologically treatedprior to the filtration of the mixed water using the clarifier, in themixed water treatment step.
 8. The sea water desalinating methodaccording to claim 3, wherein the mixing volume ratio of diluent waterto sea water is 0.1 or more when sea water=1, in the mixing step.
 9. Thesea water desalinating method according to claim 3, wherein sea water isfiltered by using a clarifier and the sea water subjected to filtrationis mixed with diluent water in the mixing step.
 10. A sea waterdesalinating apparatus for desalinating sea water by way of filtrationusing a reverse osmosis membrane device, comprising a mixed watertreatment part that mixes, as diluent water, biologically treated water,which is produced by biologically treating organic wastewater, into seawater having a salt concentration of 1.0 to 8.0% by mass to producemixed water, and feeds the mixed water produced by the mixing to thereverse osmosis membrane device, at which the mixed water is filtered.11. A sea water desalinating method for desalinating sea water by way offiltration using a reverse osmosis membrane device, comprising carryingout a mixing step of mixing sedimentation treated water that issupernatant water produced by sedimentation and separation of inorganicwastewater into sea water to produce mixed water, and a mixed watertreatment step of feeding the mixed water produced by the mixing step tothe reverse osmosis membrane device, at which the mixed water isfiltered, thereby desalinating sea water.
 12. The sea water desalinatingmethod according to claim 11, further comprising carrying out awastewater treatment step of subjecting inorganic wastewater toprecipitation and separation to produce sedimentation treated water,filtering the sedimentation treated water by using a clarifier that hasat least one of a sand filtration means, a microfiltration membrane andan ultrafiltration membrane to produce permeate, and filtering thepermeate using a reverse osmosis membrane device to produce permeatethat is purified water and concentrated water, wherein the sedimentationtreated water used as the diluent water in the mixing step is theconcentrated water.
 13. The sea water desalinating method according toclaim 11, wherein the mixed water is filtered by using a clarifier thatincludes at least one of a sand filtration means, a microfiltrationmembrane and an ultrafiltration membrane, prior to the filtration usingthe reverse osmosis membrane device, in the mixed water treatment step.14. The sea water desalinating method according to claim 11, wherein themixing volume ratio of diluent water to sea water is 0.1 or more whensea water=1, in the mixing step.
 15. The sea water desalinating methodaccording to claim 11, wherein sea water is filtered using a clarifierand the filtered sea water is mixed with diluent water in the mixingstep.
 16. A sea water desalinating apparatus for desalinating sea waterby way of filtration using a reverse osmosis membrane device, comprisinga mixed water treatment part that mixes, as diluent water, sedimentationtreated water that is supernatant water produced by sedimentation andseparation of inorganic wastewater, into sea water to produce mixedwater, and feeds the mixed water produced by the mixing to the reverseosmosis membrane device, at which the mixed water is filtered.
 17. A seawater desalinating method for desalinating sea water by way offiltration using a reverse osmosis membrane device, comprising a mixingstep of mixing, as diluent water, inorganic wastewater into sea water toproduce mixed water, and a mixed water treatment step of feeding themixed water produced by the mixing step to the reverse osmosis membranedevice, at which the mixed water is filtered, thereby desalinating thesea water.
 18. A sea water desalinating apparatus for desalinating seawater by way of filtration using a reverse osmosis membrane device,comprising a mixed water treatment part that mixes, as diluent water,inorganic wastewater into sea water to produce mixed water, and feedsthe mixed water produced by the mixing to a reverse osmosis membranedevice, at which the mixed water is filtered.
 19. A fresh watergenerating apparatus comprising a first treatment part that separateslow salt concentration wastewater having a salt concentration lower thansea water into permeate and concentrated water by way of reverse osmosismembrane filtration, and a second treatment part that mixes theconcentrated water produced at the first treatment part into sea waterto produce mixed water and separates the mixed water into permeate andconcentrated water by way of reverse osmosis membrane filtration,thereby producing fresh water separated as the permeate respectively atthe first and second treatment parts, wherein the first treatment partincludes a first salt concentration measurement means for measuring thesalt concentration of the low salt concentration wastewater, such thatthe amount of permeate produced at the first treatment part and theamount of permeate produced at the second treatment part are controlledbased on the measured value by the first salt concentration measurementmeans.
 20. The fresh water generating apparatus according to claim 19,wherein control is made such that the generating amount at the firsttreatment part is increased while the generating amount at the secondtreatment part is decreased, when the measured value is not more than,or less than, a predetermined reference value.
 21. A fresh watergenerating method comprising a first treatment step of separating lowsalt concentration wastewater having a salt concentration lower than seawater into permeate and concentrated water by way of reverse osmosismembrane filtration, and a second treatment step of mixing theconcentrated water produced by the first treatment step into sea waterto produce mixed water and separating the mixed water into permeate andconcentrated water by way of reverse osmosis membrane filtration,thereby producing, as fresh water, permeate at each of the steps,wherein the salt concentration of the low salt concentration wastewateris measured, and the amount of permeate produced by the first treatmentstep and the amount of permeate produced by the second treatment stepare controlled based on the measured value.
 22. A fresh water generatingapparatus comprising a first treatment part that separates low saltconcentration wastewater having a salt concentration lower than seawater into permeate and concentrated water by way of reverse osmosismembrane filtration, and a second treatment part that mixes, as diluentwater, the concentrated water produced at the first treatment part intosea water to produce mixed water and separates the mixed water intopermeate and concentrated water by way of reverse osmosis membranefiltration, thereby producing permeate as fresh water separatedrespectively at the first and second treatment parts, wherein the firsttreatment part includes a flow rate measurement means for measuring theinflow rate of the low salt concentration wastewater flown into thefirst treatment part, such that the filtration rate at the firsttreatment part and the filtration rate at the second treatment part arecontrolled based on the measured value by the flow rate measurementmeans.
 23. The fresh water generating apparatus according to claim 22,wherein the first treatment part and the second treatment part eachinclude plural reverse osmosis membrane units for carrying out reverseosmosis membrane filtration, and control is made such that the number ofthe reverse osmosis membrane units for carrying out reverse osmosismembrane filtration at the first treatment part and the second treatmentpart can be controlled based on the measured value by the flow ratemeasurement means.
 24. The fresh water generating apparatus according toclaim 23, wherein the control is made such that, when the measured valueis increased, the number of the reverse osmosis membrane units to carryout reverse osmosis membrane filtration at the first treatment part isincreased while the number of the reverse osmosis membrane units tocarry out reverse osmosis membrane filtration at the second treatmentpart is decreased.
 25. A fresh water generating apparatus comprising afirst treatment part that separates low salt concentration wastewaterhaving a salt concentration lower than sea water into permeate andconcentrated wastewater by way of reverse osmosis membrane filtration,and a second treatment part that mixes, as diluent water, theconcentrated water produced at the first treatment part into sea waterto produce mixed water and separate the mixed water into permeate andconcentrated water by way of reverse osmosis membrane filtration,wherein a part of low salt concentration wastewater of the firsttreatment part is bypassed to be fed as diluent water into sea water atthe second treatment part, thereby producing permeate as fresh waterseparated respectively at the first and second treatment parts, whereinthe first treatment part includes a flow rate measurement means formeasuring the inflow rate of low salt concentration wastewater flowninto the first treatment part, such that the amount of the low saltconcentration wastewater to be bypassed can be controlled based on themeasured value by the flow rate measurement means.
 26. A fresh watergenerating method comprising carrying out a first treatment step ofseparating low salt concentration wastewater having a salt concentrationlower than sea water into permeate and concentrated water by way ofreverse osmosis membrane filtration, and a second treatment step ofmixing, as diluent water, the concentrated water produced in the firsttreatment step into sea water to produce mixed water and separate themixed water into permeate and concentrated water by way of reverseosmosis membrane filtration, thereby producing permeate as fresh waterseparated respectively in the first and second treatment steps, whereinthe amount of low salt concentration wastewater to be treated ismeasured, such that the filtration rate of the first treatment step andthe filtration rate of the second treatment step are controlled based onthe measured value.
 27. A fresh water generating method comprisingcarrying out a first treatment step of separating low salt concentrationwastewater having a salt concentration lower than sea water intopermeate and concentrated water by way of reverse osmosis membranefiltration, and a second treatment step of mixing, as diluent water, theconcentrated water produced at the first treatment part into sea waterto produce mixed water and separate the mixed water into permeate andconcentrated water by way of reverse osmosis membrane filtration,thereby producing permeate as fresh water separated respectively in thefirst and second treatment steps, wherein the amount of low saltconcentration wastewater to be treated is measured, and control is madesuch that the a part of the low salt concentration wastewater is mixedinto sea water to dilute the sea water in the second treatment step. 28.The sea water desalinating method according to claim 4, wherein seawater is filtered using a clarifier and the filtered sea water is mixedwith diluent water in the mixing step.
 29. The sea water desalinatingmethod according to claim 5, wherein sea water is filtered using aclarifier and the filtered sea water is mixed with diluent water in themixing step.
 30. The sea water desalinating method according to claim12, wherein sea water is filtered using a clarifier and the filtered seawater is mixed with diluent water in the mixing step.